Symposium EQ02—Heterostructures of Various Dimensional Materials

The observation of interlayer excitons (ILE), where the electron and hole are confined in different layers, in van der Waals (vdW) type II heterostructures has ignited a new interest in investigating the optical properties of these 2D semiconducting materials. Herein, using GW and Bethe−Salpeter equation simulations, we demonstrate the generation of linearly polarized, anisotropic intra- and interlayer excitonic bound states in the transition metal monochalcogenide (TMC) GeSe/SnS vdW heterostructure. The puckered structure of TMC results in the directional anisotropy in the band structure and in the excitonic bound state. Upon the application of compressive/tensile biaxial strain, a dramatic variation in excitonic energies, the indirect-to-direct semiconductor transition, and the red/blue shift of the optical absorption spectrum are observed. The variations in excitonic energies and optical band gap have been attributed to the change in effective dielectric constant and band dispersion upon the application of biaxial strain. The generation and control over the interlayer excitonic energies will find applications in optoelectronics and optical quantum computers and as a gain medium in lasers.

We report MOVPE growth of h-BN on sapphire substrates pre-patterned with SiO2 and SiN with the goal of investigating the possibility of new kinds of selective area growth. We see that after h-BN growth, the GaN-based device structures grow selectively on sapphire but not on SiO2 and SiN. This result is surprising because BN is not anticipated to exhibit strong selectivity on dielectric patterns.

To understand how this happens, 20 nm thick h-BN layers were grown separately on dielectric patterned sapphire substrates. On sapphire, the h-BN formed the typical semi-hexagonal wrinkles which is evidence of growth of high quality single crystalline 2D h-BN layers. On SiO₂, localized SIMS measurements confirmed the deposition of BN, so as expected there is not selective area growth of BN. Additional cross sectional TEM studies confirm that the h-BN on the sapphire substrates is highly oriented and layered whereas the BN on the dielectric patterns are nanocrystalline and randomly alligned. Given the lack of crystalline order on the surface of the SiO₂ and SiN regions, the presence of disordered BN there is not surprising.

We have performed several experiments growing functional GaN-based LED structures after h-BN growth during the same growth run, and the selective growth of the GaN-based structures on the h-BN/sapphire is reliable. TEM measurments show that homogeneous, single crystalline GaN based device structure growth occured only in regions where layered h-BN is deposited, and little to no GaN growth occurred on the randomly-oriented BN on SiO₂. SEM and cathodoluminescence studies confirm that the GaN regions are separated from each other by the dielectric patterns. We conclude from further structural and optical charcaterisations that the degree of order in BN deposits controls the selectivity of III-Nitride device structures. It seems that the structure of the BN surface rather than its chemical composition determines whether or not GaN-based materials can grow. Compared to unpatterned areas, fewer GaN dislocations were found in patterned areas, and Spontaneous delamination of III-nitride device structures on h-BN, a problem with full wafer device structures on h-BN, was avoided. The results allow for localized device liftoff and transfer from h-BN since the devices are completely physically isolated from eachother. The possibility of pixel by pixel selective transfer opens up heterogeneous integration possibilities needed for realization of novel device architectures and device isolation and device passivation.

Hexagonal boron nitride (h-BN) is a two dimensional wide-bandgap semiconductor, which can be used as a release layer for mechanical lift-off of as-grown III-nitride heterostructures and their processed devices such as LEDs and HEMTs. However, it has been challenging to control the strain in this layered material which results in spontaneous delamination during epitaxial growth or front-end processing of these devices, mainly for large surface areas. Here, we report a unique approach of controlling the adhesion of this layered material, which can result in either desired lift-off from layered h-BN or mechanically inseparable robust h-BN layers. We found that the growth conditions of an AlN buffer layer on top of h-BN plays an important role for controlling the adhesion of the subsequent device structure grown on h-BN. AlN layer grown above a certain temperature leads to the Al diffusion into h-BN, which enhances the surface and interface dangling bonds, and anchors the layered h-BN. The effect of the Al concentration into h-BN layers on the mechanical lift-off is verified by studying carefully BAlN alloys with different Al content. It has been observed that above a certain Al content, the layered BAlN sample cannot be lifted-off, supporting our investigation. In addition, we provide the flexible approach on the control of the lift-off process by variation of the thickness of h-BN. Furthermore, we illustrate the potential of this investigation by controlling the mechanical lift-off of two types of device structures (LED and HEMT). These results extend the control of the adhesion of 2D/3D materials, opening the route to large-scale fabrication of III-N devices grown on h-BN, and bringing them close to industrial applications.

The instructor will discuss about new epitaxy lift-off technique so called remote epitaxy that can produce freestanding semiconductor membranes. Discussion will include 1) Remote epitaxy mechanism 2) High yield peeling mechanism 3) Reusability of the substrates 4) Economic aspect of remote epitaxy 5) Heterointergration of 3D materials with 2D materials for advanced heterostructuring.

The instructor will provide the overview regarding the epitaxial growth of Nitride-based semiconductor materials by MOVPE including GaN and h-BN, and design device structures for opto-electronic devices. In addition, Instructor will cover the fabrication of nanostructures and materials characterization. The epitaxial growth of 3D and 2D materials and characterization of them is the basis of fabrication of heterointegration system for various device applications.

Heterostructures built on three-dimensional (3D) semiconductor materials are rooted from the theory conceived by Nobel Laureate Hebert Kroemer in 1957. Since then, the four classes, namely, GaAs-, InP-, Si/Ge-, and III-nitrides-based heterostructures, have revolutionized the electronics and optoelectronics that set the foundation of today’s communications, lighting, sensing, infotainment, etc. These heterostructures were universally formed by epitaxy techniques uniquely governed and enabled by the stringent requirements of lattice matching. Limited by this requirement, realizing heterostructures between lattice-mismatched semiconductors has been a decades-long obstacle, although the materials space to explore and the number of heterostructures that can be made are vast compared to its lattice-matched counterparts. In this tutorial, lattice-mismatched heterostructures, i.e., heterogeneous heterostructures, or arbitrary heterostructures, formed via semiconductor grafting based on a quantum tunneling gluing approach are described. The tutorial will include the history of attempts on forming lattice-mismatched heterostructures, physical principles of bypassing the lattice constraints, fabrication methods and scalability, and a set of application examples that are selected from about 18 pairs of heterostructures formed between Si, Ge, GeSn, GaAs, InGaAs, AlGaAs, GaN, AlGaN, SiC, GaN, Diamond, β-Ga2O3, and CsPbBr3.

The isolation of a growing number of two-dimensional (2D) van der Waals (vdW) materials has inspired worldwide efforts to integrate distinct 2D materials into vdW heterostructures. Over the past decade a tremendous amount of research activity has occurred in assembling disparate 2D materials into “all-2D” van der Waals heterostructures and thereby making outstanding progress on fundamental studies at 2D/2D interfaces. However, practical applications of 2D and other low-dimensional materials will require a broader integration strategy. Namely integration of 2D materials with other self-passivated, quantum-confined or molecular materials will be necessary. In this regard significant progress has been achieved in recent years, which involve successful integration of 2D materials with organic small molecules, quantum-dots, nanocrystals, semiconducting polymers, inorganic nanowires as well as carbon-nanotubes. These studies have produced interesting results both in terms of basic science as well as device applications including in photodetectors, logic, memory and light emitting devices. In addition, 2D van der Waals layers have also provided a rich and interesting avenue in terms of hetero-integration with three-dimensional (3D) and bulk materials including 3D semiconductors, piezoelectrics, ferroelectrics and magnets. The 2D/3D combinations are particularly favorable and attractive in terms of vertical integration on Silicon CMOS and enabling “More than Moore” approaches including memory, electronic-photonic integration etc. for semiconductor chips. In this tutorial, I will provide an overview of the progress on the above described heterostructures and their devices focusing on the unique advantages and capabilities that each materials combination provides. I will end with a broad and forward-looking perspective on future investigation and development opportunities in mixed-dimensional heterostructures and their applications for the research community.

The heterogeneous integration of dissimilar materials is a long pursuit of material science community and has defined the material foundation for modern electronics and optoelectronics. The typical material integration approaches usually involve strong chemical bonds and aggressive synthetic conditions and are often limited to materials with strict structure match and processing compatibility. Alternatively, van der Waals integration, in which freestanding building blocks are physically assembled together through weak van der Waals interactions, offers a bond-free material integration strategy without lattice and processing limitations. In this tutorial, I will discuss the fundamental forces involved in van der Waals integration and generalize this approach for flexible integration of highly disparate materials to produce artificial heterostructures with minimum interfacial disorder and to enable high-performing devices.
I will start with a brief introduction of some early background on using this approach for damage-free dielectric integration to enable high speed transistors from atomically thin graphene and 2D semiconductors, and then focus my most of my discussion on our recent efforts in exploring this approach for low-energy metal integration to enable high performance contacts for delicate materials. In particular, I will highlight our recent advancement in exploiting the van der Waals integration approach for seamless integration of conventional metal thin film electrodes with the 2D semiconductors to create nearly ideal metal-semiconductor junctions essentially free of interface states and Fermi level pinning, for the first time the experimental achieving Schottky-Mott limit that has been difficult to achieve in Schottky diodes, and realizing the 2D diode approaching the intrinsic exciton physics limit. Next, we further extends this approach for damage-free integration of metal contacts on delicate halide perovskite thin films, realizing high performance van der Waals contacts with an atomically clean contacting interface and greatly reduced contact resistance, for the first time allowing systematic electrical transport studies of halide perovskites down to cryogenic temperatures and establishing technical foundation towards unraveling the complex transport behavior and exploring new physics in this unique class of “soft-lattice” materials.
These advancements highlight the unique merit of the bond-free van der Waals integration approach in creating pinning-free heterostructure interfaces. I will conclude with a brief perspective on these strategies for creating diverse artificial heterostructures as a versatile material platform with electronic structure by design to unlock new physical limits and enable a new generation of devices with unprecedented performance or unique functions, as illustrated in our recent effort including a new class of high-order vdW superlattices integrating highly distinct atomic crystals and molecular layers, solution processable vdW thin electronics, hybrid vertical transistors and low voltage memoristors, programmable devices from 2D/superionic solid vdW heterostructures sensitive pressure sensors and electronic skins, highly reliable molecular tunneling transistors.

We exploit defects in a 2D barrier layer to drive the seeded lateral epitaxy of atomically smooth, exfoliatable, single-crystalline membranes [1]. This growth mode offers many of the advantages of the recently discovered remote epitaxy, including novel strain relaxation pathways and the ability to exfoliate free-standing membranes [2], plus additional advantages of (1) tolerance for imperfect graphene/substrate interfaces and (2) the ability to engineer the growth by patterning the 2D graphene layer. We show that GaSb films grow on graphene-terminated GaSb (001) via a seeded lateral epitaxy mechanism, in which pinhole defects in the graphene serve as selective nucleation sites, followed by lateral epitaxy and coalescence into a continuous film. Importantly, the small size of the pinholes permits exfoliation of a continuous, free-standing GaSb membrane. By combining molecular beam epitaxy with in-situ electron diffraction and photoemission, plus ex-situ atomic force microscopy and Raman spectroscopy, we track the graphene defect generation and GaSb growth evolution a few monolayers at a time. Our discovery provides a highly tunable method to engineer single-crystalline 3D materials on precisely tuned 2D barriers.

[1] S. Manzo, et. al., arXiv:2106.00721 (2021).
[2] D. Du, et. al., Nature Communications 12, 2494 (2021).

In conventional epitaxy of single-crystalline semiconductor materials, it is challenging to separate the grown layer with the substrate due to the strong bonding at the interface. Remote epitaxy is a recently discovered method to grow single-crystalline thin films on graphene, wherein the grown film can be exfoliated at the graphene interface to form freestanding membranes.
Here, we present our recent development on remote epitaxy of III-V semiconductors. We show that directly growing 2D materials on III-V substrates as a remote epitaxy template is an ideal pathway that can eliminate transfer process-related defects and can realize wafer-scale process of remote epitaxy and substrate reuse. We present the strategies to grow 2D materials on the surface of III-V materials, which is much more challenging than thermally robust substrates such as SiO2/Si or sapphire. The nucleation of III-V on 2D material-coated III-V platforms via remote interaction is investigated both experimentally and theoretically. Lastly, we show advanced remote epitaxial platforms and optoelectronic applications enabled by remote epitaxy, and the capability to recycle the III-V substrates for repeated remote epitaxy and production of freestanding III-V thin films.

Indium phosphide (InP) is a III-V semiconductor that offers outstanding photonic properties that outperforms silicon, but the cost of InP wafers is extremely expensive. Although reusing original wafers can effectively minimize the cost of wafers, current techniques for wafer recycling of InP add significant costs in fabrication, nullifying the cost savings by reusing the wafers. Besides, unlike other III-V materials such as gallium arsenide (GaAs), commonly used epitaxial lift-off method for wafer recycling is not well studied for InP due to lack of latticed matched sacrificial layers, which makes reusing InP wafers more difficult. Remote epitaxy is a newly discovered method that enables single-crystal growth of III-V semiconductor thin films and easy exfoliation of the grown film, thus promising for a new cost-effective pathway of reusing InP wafers. However, previous methods of transferring 2D materials onto InP wafers, which use polymethyl methacrylate (PMMA) or metal stressor layers to transfer 2D materials grown on foreign substrates like copper or silicon carbide, introduce defects and damages on the 2D layer and/or substrates during the transfer process. Remote epitaxial films grown on the damaged 2D layer/substrate suffer from lower crystal quality and imperfect exfoliation, which undermines wafer reusability and device performance.

Here we report the MBE growth of amorphous boron nitride (a-BN) on InP wafers at low temperature that enabled improved quality of remote epitaxial films and their perfect exfoliation. We show fully covered a-BN on InP substrates despite phosphides' low decomposition temperatures. The surface of a-BN coated InP substrate remains smooth with a RMS roughness of around 3Å. We also demonstrate 100% coverage of single-crystalline InP thin films grown on a-BN, with the film's quality significantly improved compared to the case of transferred 2D materials. In addition, roughness of the substrate's surface remains the same after exfoliation of grown InP film, and the growth and exfoliation were successfully repeated multiple times, proving the feasibility for InP wafer recycling. Through this low temperature MBE growth approach and remote epitaxy, we successfully demonstrate large-scale flexible thin film exfoliation and recycling of InP substrates, which will lead to new opportunities in InP thin film-based photonics and novel heterostructures with significantly reduced cost.

High-quality single-crystalline thin films are a significant factor for achieving substantial electrical and optical properties of various dimensional materials. Heterogeneous integration of these films may be achieved by in-situ fabrication or lift-off technologies from compliant substrates with remote and van der Waals epitaxy growth of semiconductors.[1] However, the quality of transferred semiconductor films depends on the quality and real-size of manually transferred two-dimensional (2D) materials, limiting the size of direct large-area single-crystal films.

In this presentation, a one-step growth method will be introduced to fabricate thin films that are incommensurate to their substrate through a spontaneous van der Waals-like epitaxy.[2] In this low-temperate growth, a buffer layer of chalcogenide is spontaneously attached to the interface with a weak chemical interaction between a surface-reconstructed substrate and a 3D film. An epitaxial relationship between the substrate and film is maintained across this interfacial layer. Moreover, no mismatch dislocations are observed at the interface, which allows high-quality single-crystal thin film growth. This unique interface complexion allows for the lift-off of films, such as CdTe and InSb, without first transferring a 2D material onto their growth substrate for remoting epitaxy.

The presentation will focus on the fundamental interface understanding of cadmium telluride (CdTe) on a sapphire, fabricated via pulsed laser deposition at approximately 300 °C.[2, 3] A monolayer of chalcogenide (tellurium) at the interface is examined by various spectroscopic scanning transmission electron microscopy (STEM) techniques, including low-loss and core loss electron energy-loss spectroscopy (EELS) to reveal both the bonding and chemical composition at the interface. The CdTe film is investigated at the bulk-scale using 3D x-ray diffraction measurements and the atomic-scale using STEM imaging of the interfacial and CdTe film regions. The strain at the CdTe-sapphire interface is examined using both geometric phase analysis (GPA) and the STEM-moiré analysis. Microscopic investigations provide evidence that strain-free CdTe films grow with high quality on the Tellurium-terminated sapphire surface. The relatively low growth temperature maintains the top-most sapphire layer to the single aluminum reconstruction, as confirmed by core-loss EELS. Tellurium is more kinetically favorable to nucleate above oxygen sites in sapphire than Cadmium at these growth and interface conditions. This is followed by the CdTe film with its first tellurium layer oriented toward the pseudomorphic tellurium layer on the sapphire forming van der Waal-like bonds. This interface complexion allows for a unique incommensurate van der Waals heteroepitaxy growth.

Finally, the presentation will show the realization of a monolayer Tellurene, a 2D form of tellurium with a 3-atom thick, if more tellurium is deposited at the film/substrate interface. The growth progress of this interface complexion could lead to the fabrication of various dimensional (3D/2D/3D) high-quality heterostructures via one-step growth using a pulsed laser deposition system.

References
[1] S-H Bae et al., Nature Nanotechnology 15 (2020), p. 272.
[2] SM Jovanovic et al., Small 16 (2020), p. e2004437.
[3] H El-Sherif et al., Microsc microanal 26 (2020), p. 1.

Transition metal dichalcogenides (TMDs) are 2D semiconductors characterized by unique features such as layer-dependent direct/indirect band gap transition, high spin-orbit coupling and strain modulation [1]. Depending on the number of layers, their band gap can vary within the visible/near-infrared range making TMDs suitable for a wide variety of optoelectronic applications [2]. In particular, the possibility to induce the local emission of photons by applying strain is highly beneficial for the design of quantum emitters [3]. In this context, the combination with 1D nanostructures offers an elegant solution as the dimensionality mismatch between ultrathin TMDs and nanowires results in a localized strain [4]. For these reasons, the integration of MoS₂ with compound semiconductor nanowires in the form of a heterostruture is expected to widen the functionalities of the ensemble. Selective area epitaxy (SAE) can be used to grow ordered arrays of III-V nanowires. Due to the small footprint, nanowires with very low defect density can be grown on a large number of substrates [5]. Additionally, III-V nanowires can act as natural waveguides increasing the directionality of the photon emission [6].
In this work, we report on the fabrication of heterostructures formed by exfoliated mono- and bi-layers MoS₂ and GaAs nanowires (NWs) and preliminary optical characterization of the ensemble. Self-catalysed vertical GaAs NWs were grown by SAE on Si (111) substrate, with a diameter of 150nm and height of 400nm. The exfoliated flakes were transferred on the NWs using an all-dry viscoelastic stamping method. Atomic force microscopy (AFM) was used to get feedback on the integration. AFM analysis indicates that the monolayer is stable on top of the NWs in most of locations, although a non-zero possibility of piercing events exists. Raman analysis evidenced a small shift of the in-plane (E_2g) mode at low temperature. Photoluminescence (PL) spectra on bare MoS₂ showed the existence of A exciton and trion peaks as well as a large band peaked around 1.7 eV, attributed to sulfur vacancies [7]. PL analysis on MoS₂/GaAs NWs highlights an enhanced intensity of the same peaks. PL micromapping was used to visualize the increase in emission intensity within the array. The defect peak centered around 1.7 eV shows a non-uniform intensity enhancement whereas the direct bandgap emission from MoS₂ is uniformly enhanced across the nanowire array.

References
[1] Manzeli et al., Nat.Rev.Mater. 2, (2017)
[2] Mak et al., Nat.Photonics. 10, (2016)
[3] Branny et al., Nat.Commun. 8 (2017)
[4] Mukherjee et al., Nat. Commun. 11, 5502 (2020)
[5] Glas et al., Phys. Rev. B 74, 121302(4) (2006)
[6] Frederiksen et al., ACS Photonics 4, (2017)
[7] S. Tongay et al., Sci. Rep.. 3, (2013)

Remote epitaxy (RE) is a novel growth process performed on graphene covered substrates where adatom registry is guided by the partially-screened electrostatic fields from the underlying substrate rather than interactions with the graphene lattice. The graphene acts as a spacer, allowing the ease of epitaxial layer removal and transfer to desired substrates. This RE technique has the possibility of impacting many research areas including SiC and GaN SMART power, integration of opto- and electronic devices, HEMT performance, flexible electronics and quantum sciences.

In this work, we focus on the RE of SiC, which is directly grown on epitaxial graphene (EG) on SiC(0001) using a chemical vapor deposition (CVD) reactor. The graphene is grown via the Si sublimation of SiC. Both on-axis (6H-SiC) and off-axis (4H-SiC cut 4° off-axis towards the [11-20]) substrates were investigated where the ability to maintain this graphene at elevated growth temperatures in a SiC CVD environment is crucial for the success of remote epitaxy. Following epitaxial graphene growth, SiC growth was performed and the growth parameters were modified to reduce SiC polytype conversion and other extended defects. Films were characterized using scanning electron microscopy, transmission electron microscopy and electron backscattering detection. To evaluate the ability to grow SiC remote epitaxially, a Ni metal stressor technique was used to exfoliate the layers and demonstrate exfoliation yields up to 50 – 70 %.

This work was supported by the U.S. Office of Naval Research.

Remote epitaxy – an emerging growth method of single-crystalline membranes copied from underlying substrates through atomically thin graphene interlayer – has greatly expanded the material spectrum by efficiently producing costly thin films or heterogeneously integrating unique nanosystems. It was recently demonstrated that slippery graphene promotes a strain relaxation of epitaxial film on its surface, which also allows the heteroepitaxy of lattice-mismatched systems with reduced defect density. Despite such benefits, remote epitaxy has its fundamental limitations in non-polar materials because the ionicity of materials strongly governs a distant atomic interaction between the substrate and epitaxial layer. Thermally or chemically weak materials that are incompatible with graphene transfer or growth on them also restrict their remote epitaxy. Here, we introduce periodic openings in graphene to induce a lateral overgrowth of thin films, where an opening acts as a favorable nucleation site allowing the epitaxy to occur regardless of polarity and process compatibility of materials. Nanostructured graphene interlayer enables a facile exfoliation of epitaxial films from substrate as well as a spontaneous relaxation of misfit strain during heteroepitaxy depending on its structural parameters such as period and size of openings. Engineered graphene is, thus, expected to provide a universal epitaxy platform that can accommodate the growth, release, and transfer of a wider variety of high-quality membranes.

GaN has been widely used for various applications including power electronics, RF devices and optoelectronic applications. Recently, remote epitaxy technology has been introduced, a substrate below 2D materials also plays a role to intervene in the epitaxial growth process, leaving room to realize a novel epitaxial growth process. Potential fluctuation—the difference between potential energy maxima and minima along the surface of the substrate—is influenced by this remote interaction; the fluctuation is predominant over the vdW force, due to the atomic thickness of 2D materials. The remote interactions can be controlled by modulation of polarities of 2D materials and epitaxy materials. Polar-material-based heterostructure, such as GaAs, InP, GaP, and GaN with their substrate sandwiching 2D materials, enables the semiconductor film to be exfoliated from the underlying substrates through the 2D materials. This process is expected to open a novel avenue for the field of non-silicon electronics and photonics, where the ability to re-use graphene-coated substrates allows savings on the high cost of non-silicon substrates.

One of the frontiers of mixed dimensional heterostructure design lies in the ability to grow thin epitaxial films of conventional three-dimensional (3D) metals or semiconductors on two-dimensional (2D) materials. The structure and alignment at this 2D/3D interface affects properties, such as electron transport, contact resistance, and photo-current, of devices that utilize 2D materials.¹ Moreover, integration of 2D/3D heterostructures can result in unique (opto-)electronic, magnetic, thermal, or mechanical properties of the combined heterostructure not present in individual components.^2,3

Here, we report the direct epitaxial growth of ultrathin titanium/gold (Ti/Au) heterostructures on suspended 2D materials. The Ti/Au system is important due to Ti’s continued use as an adhesion layer across a variety of applications in micro- and nano- electronics. Moreover, ultrathin (<1nm) Ti/Au layers exhibit improved thermal stability, plasmonic response, and superconducting transition temperature compared to thicker (>2nm) layers. We perform epitaxial metal growth on suspended 2D materials substrates in ultra-high vacuum (UHV) and monitor the structure evolution during annealing using in-situ transmission electron microscopy (TEM). We find that ultrathin (<1nm) epitaxial Ti nanoislands can be grown on graphene at high temperatures. The island geometry exhibits a large dependence on the layer number of the 2D material, and on thin 2D layers the Ti is extremely thin and compliant to the substrate. Subsequent Au deposition without breaking vacuum shows 100% selectivity on these ultra-thin Ti layers, and the Au completely wets the surface instead of forming conventional Volmer-Weber islands. In contrast, the Au forms islands when the Ti layer is exposed to oxygen at 10^-6 Torr before Au deposition. Moreover, we observe greatly improved thermal stability against dewetting of the Ti/Au layer up to 1000^oC. Electron energy loss spectroscopy (EELS) evidence that the thin Au capping layer prevents the Ti from oxidation. We discuss the potential of these Ti layers as a general strategy for creating ultrathin bottom-up 2D/3D heterostructures through epitaxially anchoring other metallic or semiconducting layers on 2D materials, and consider routes towards post-oxidation of the layers to form thin oxide/2D interfaces. This opens up a new set of possibilities for the heterointegration of 2D materials and thin epitaxial 3D films, adding to the toolbox of multidimensional heterostructure growth and design.

References
1. Jariwala, D., Marks, T. J. & Hersam, M. C. Mixed-dimensional van der Waals heterostructures. Nat. Mater. 16, 170–181 (2017).
2. Kim, Y. et al. Remote epitaxy through graphene enables two-dimensional material-based layer transfer. Nature 544, 340–343 (2017).
3. Reidy, K. et al. Direct imaging and electronic structure modulation of moiré superlattices at the 2D/3D interface. Nat. Commun. 12, 1290 (2021).

Semiconductor-superconductor hybrid structures offer an attractive platform to explore interesting low temperature transport phenomenon. In particular, one dimensional semiconducting nanowires with spin orbit coupling proximitized by a s-wave superconductor has been proposed to host Majorana Fermions, relevant for topological quantum computing. Selective area growth offers a scalable route to growing and fabricating complex networks of such semiconductor superconductor nanostructures. However selective area grown in-plane semiconductor nanowires often suffer from a high density of defects due to lattice and symmetry mismatch between the nanowire material and the substrate. This leads to poor electrical characteristics such as low electron mobility and low coherence lengths. Heterostructures can potentially solve this problem by incorporating buffer layers to reduce lattice mismatch and using capping layers to reduce surface scattering. Further, a clean interface between the superconductor and the semiconductor is crucial for inducing a hard proximitized superconducting gap in the nanowire. Since post-growth superconductor etching can often introduce defects in the semiconducting nanowire, an in-situ selective superconductor deposition can prove to be extremely useful in this respect.
In this talk, we discuss selective area growth of III-V nanowires on InP(001) substrates using chemical beam epitaxy. Specifically, we grow InP/InGaAs/InP quantum wells in in-plane nanowire geometry with lattice matched (In_0.53Ga_0.47As) and strained (In_0.75Ga_0.25As) channels that are 20nm thick. Cross-sectional scanning transmission electron microscope (STEM) imaging of the grown nanowires shows abrupt defect-free interfaces. In addition, we show growth of InAs nanowires with graded InGaAs buffers. Effects of InGaAs buffer on growth morphology are studied using high resolution cross-sectional STEM analysis. To grow superconductor islands on the in-plane nanowires, we fabricate high aspect ratio SiO₂ shadow walls positioned within 200nm of the nanowires. Together with angled low-temperature deposition, this enables in-situ growth of superconductor islands on the in-plane nanowires at predetermined positions, while maintaining the pristineness of the interface - as confirmed by the proximity induced superconductivity in the nanowire. Superconductor-normal-superconductor junctions with sub 100nm normal junction lengths are achieved with tin as the superconductor on InAs nanowires. By incorporating different shapes of shadow walls, positioned close to the nanowire trench, we show that various semiconductor-superconductor hybrid geometries can be directly grown without breaking vacuum. Finally, we discuss magneto-transport measurements on these hybrid nanostructures.

Next-generation electronics will expect new functionalities such as high performance, low power consumption, lightweight, flexibility, conformality, and self-power. In this regard, demand for new heterostructures has been getting high. In this talk, I will discuss how we can produce freestanding 3D materials and 2D materials that are stepping stones to realize new heterostructures. As they are a new form of heterostructures, new architectures for functional devices and physical phenomenon can be realized. For freestanding 3D materials, we have studied single crystalline growth on graphene-coated substrates. As graphene has lattice transparency, single-crystalline materials were successfully grown. Because the interface between grown single crystals and graphene-coated substrates has substantially small binding energy, the grown single-crystalline materials can be exfoliated to produce freestanding 3D materials. For freestanding 2D materials, we have conceived a unique idea that selectively separates monolayer 2D materials from multilayers, named layer-resolved splitting, thereby multiple monolayer 2D materials are harvested from multilayer 2D materials. With such approaches, 2D and 3D mixed dimensional heterostructures have been developed for new physical coupling phenomena and new device architectures.

Reference
[1] Sang-Hoon Bae, et al Nature Nanotechnology, 15, 272–276 (2020)
[2] Sang-Hoon Bae, et al Nature Materials, 18, 550–560, (2019)
[3] Jaewoo Shim, Sang-Hoon Bae et al Science, 362, 6415, 665-670, (2018)

Mixed-dimensional heterojunctions (MDHJs) are an emerging class of nanoscale devices whose electrical and optical properties reflect the nature of their constituent materials. The prospect of mixed-dimensional van der Waals transistor architectures (1D/2D or 2D/3D) may offer substantial advantages in both the ease of fabrication and performance of the realized devices. For example, bulk p–n junctions usually need diffusion or implant-anneal sequences to achieve the necessary doping profiles, while these steps may be bypassed entirely by using synthetic van der Waals MDHJs.

In this work, we demonstrate ultra-downscaled MDHJ diodes and transistors based on the van der Waals heterojunction formed between monolayer n–type MoS₂ and an individual p–type single-walled semiconducting carbon nanotube (SWCNT). We fabricate these heterojunctions using a non-deterministic wet transfer process where chemical vapor deposition (CVD)-grown monolayer MoS₂ flakes are deposited directly on top of individual CVD-grown semiconducting SWCNTs. We demonstrate that the rectification ratio of the formed individual MDHJs can be tuned by several orders of magnitude by back-gating through the SiO₂ substrate, while the transfer characteristics of the individual junctions demonstrate anti-ambipolar behavior previously observed in ensemble CNT/MoS₂ junctions.

Moreover, we utilize the MDHJ diode to fabricate atomic-scale p-type junction field-effect transistors, where the voltage applied to the MoS₂ gate can modulate the conductance through the underlying SWCNT transistor channel. Our experimental results match well with technology computer-aided-design (TCAD) device modelling, where the transistor functionality is found to be dominated by band-to-band tunneling (BTBT) across the heterojunction forming the JFET gate region. In addition, we find that in extreme bias limits, the high breakdown voltages inherent to this mixed-dimensional van der Waals interface can push the device into unusual operating regions, where the polarity of the transistor drive current can be controlled by the gate voltage magnitude.

The high exciton binding energy and mechanical flexibility of transition metal dichalcogenides (TMDs) make them attractive for next-generation photonics. However, because of their atomically thin nature, there is a challenge in obtaining sufficient absorption and emission of the incoming electromagnetic (EM) field. Plasmonic core-shell structures of TMD-encapsulated nanoparticles have demonstrated significantly improved light-matter interactions, highlighting this architecture for applications in nanophotonic and optoelectronic devices. Recently, a high index silicon dielectric nanoparticle in the core-shell architecture has been theoretically predicted to further reduce losses and enhance resonance of the near-EM fields. The resonance coupling of the magnetic dipole mode of the silicon core and exciton transitions in the TMD may thereby enable longer exciton lifetimes, yielding enhanced energy transfer and absorption/emission of the field. However, to date, this architecture has not been experimentally demonstrated.

Here, we experimentally realize this proof-of-concept of advanced Si@MoS₂ heterogeneous architecture, encapsulated via chemical vapor deposition. The size of the silicon nanoparticles, which have diameter 175±25 nm, is comparable to the wavelength of the EM field, putting this hybrid system in the Mie scattering regime. Structural and chemical analyses confirm the presence of Mo and S in the shell and Si in the core. In the core-shell architecture, we observe an enhanced photonic response relative to the bare Si nanoparticles and planar MoS₂. This distinct behavior can be explained by a displacement current loop circulating inside the dielectric Si core which leaks out to the shell and enhances the near-EM field. Here, the Si core acts as an antenna to effectively excite the TMD shell emitter. Moreover, there are two major factors that control the degree of coupling response: the core diameter and shell thickness. Tuning the size of the core has enabled us to optimize the optical resonance. Dark-field single-particle optical scattering measurements confirm the energy coupling between the core and the shell measured through Rabi splitting of the magnetic dipole resonance mode of the nanoparticle. We believe Si@MoS₂ is a technological-relevant material system and can assist to understand novel optical responses and the principles underlying these phenomena.

This work is primarily supported by the National Science Foundation (NSF) under Grant No. DMR-1929356. We made use of the EPIC and KECK II facility of Northwestern University’s NUANCE Center, which has received support from the SHyNE Resource (NSF ECCS-2025633), the IIN, and Northwestern’s MRSEC program (NSF DMR-1720139).

The goal of this work is to identify factors which modulate transitional and orientational order in 2D self-assembled superlattices using polygon shaped nanocrystal building blocks. Using a combination of transmission electron microscopy, electron diffraction, and hard particle Monte Carlo simulations, we quantify the translational and orientational order in 2D superlattices of hexagonal prism shaped CdSe/CdS nanocrystals and cube shaped CsPbBr₃ nanocrystals. Superlattices derived from cube shaped nanocrystals display less translational order compared to hexagonal prism shaped nanocrystals both experimentally and in simulations. To first order, this effect can be understood based on geometric considerations inherent to the combined rotational and translational symmetries of different polygonal shapes and their superlattice geometries. Cubes form a simple cubic lattice in which entire rows of nanocrystals can slide past each-other with little energy penalty whereas hexagons interlock in a hexagonal tiling, leading to a structure which is robust to nanocrystal translations. Nanocrystal shape also modulates the degree of orientational order, and in this case, cube assemblies display a narrower orientation distribution than hexagonal prism assemblies. An intuitive explanation for this arises when the influence of organic ligands surrounding the polygonal inorganic core is considered. A hexagonal prism displays a more ``spherical'' interaction potential compared to cubes explaining the observation to first order. The results presented here outline an intuitive framework for identifying nanocrystal superlattice structures which favor translationally versus orientationally ordered self-assembled superlattices.

As the world transition from fossil fuels to intermittent renewable energy sources, for instance, solar and wind, the demand for batteries and supercapacitors to store energy is increasing more now than ever. Exploitation of low-cost, abundant, and sustainable electrode materials in a way that not only meets this energy storage demand but also contributes to circular economy is attractive. In this study, self-doped Fe-carbon nanocomposite heterostructures for supercapacitor electrodes were successfully synthesized from abattoir waste.
X-Ray Diffractometry (XRD) show turbostratic carbon structure formation is the heterostructures. Raman spectroscopy show high degree of graphitization based on the D and G bands ratio. Fourier Transform Infrared (FTIR) measurements show oxygen containing functional groups in the iron doped carbon matrix while high resolution atomic absorption spectroscopy show significant amount of Fe in the carbon heterostructures. These electrode materials afforded a high specific capacitance of 322 Fg^-1 with 99% coulombic efficiency.

Hexagonal boron nitride (hBN) is a Van der Waals material with a 2D atomic structure similar to graphene, whose unique optical properties have raised significant interest; its wide direct band gap and robust excitons enable intense emission in the deep UV region. Additionally, the crystallographic defects in hBN form states within the band gap which can be engineered for efficient, high-brightness, quantum emission in the UV to IR range, even at room temperature [1,2]. Although extensively studied, the origin of the complex emission spectrum of hBN is still debated and mostly unknown. In this context, cathodoluminescence (CL) mapping is a powerful tool. By resolving the optical emission down to the nanoscale upon electron beam exposure, CL allows to access the optoelectronic phenomena occurring in specific locations. However, when dealing with ultrathin materials, high electron beam energies are not efficient probes as only a small fraction of the electron energy is transferred to hBN [3]. On the other hand, the use of low beam energies negatively impacts the spatial resolution.
This work explores the use of CL mapping with beam energies from below 1keV to 10keV to investigate the optoelectronic properties of hBN integrated onto different substrates. Thin flakes (3-50 nm) of hBN are exfoliated via the standard tape method and transferred on sapphire and horizontal GaAs nanomembranes (NMs) using an all-dry viscoelastic stamping method. Samples with different flake thicknesses were also fabricated to explore thickness dependent properties. Depositing the flakes onto sapphire or nanostructures allows to take advantage of the underlying substrate to increase the light emission, thus enabling the access to the optical fingerprint of the material.
GaAs NMs are grown on (111)B GaAs substrates by selective area epitaxy. These NMs present a slab morphology, whose dimensions can be tuned by modifying the growth parameters. In this study, nanostructure arrays with different heights, widths and pitches were explored. The NMs' morphology and aspect ratio are expected to dictate the strain induced on the hBN crystals as well as the contact area between membrane and hBN. Spectrally integrated CL maps evidenced that the presence of the membranes underneath the flake has an impact on the emission intensity in the whole photon energy range investigated. In particular, the emission bands at 320nm and 400nm, attributed to C/O impurities and N vacancies respectively, appear brighter in areas where there is a nanomembrane below the flake when compared to what is observed in the off-nanomembrane position. This effect is more pronounced with thicker flakes, where the light emission is more intense. Another notable variable is the topology imposed on the flake during its deposition onto the substrate. The flake can be conformal to the structures or suspended on top of them, mainly depending on the NM morphology. The results highlight how a suspended flake exhibits a stronger emission between membranes and lower emission on top of the membrane as compared to a flake conforming to the substrate. Monte Carlo simulations suggest that the topology has an impact on the interaction with backscattered electrons and therefore on the emission intensity.
References:
1. Choi, S. et al. ACS Appl. Mater. Interfaces 8, 29642–29648 (2016).
2. Koperski, M. et al. Opt. Commun. 411, 158–165 (2018).
3. Negri, M. et al. Nano Lett.20, 567–576 (2020).

The synthesis of high entropy oxide (HEO) nanoparticles (NPs) possesses many challenges in terms of process complexity and cost, scalability, producing nanoparticle morphology, and rapid synthesis. Herein, we report the synthesis of novel single-phase solid solution (Mn_, Fe_, Ni_, Cu_, Zn)₃(O)₄ quinary HEO NPs produced by flame spray pyrolysis (FSP) route. The aberration-corrected scanning transmission electron microscopy (STEM) technique is utilized to investigate the spinel crystal structure of synthesized HEO NPs and energy dispersive X-ray spectroscopy analysis confirmed the high entropy configuration of five metal elements in their oxide form within a single HEO nanoparticle. STEM analysis shows the synthesized HEO NPs are single phase solid solution spinel-structured with the lattice constant of 8.39 Å. Selected area electron diffraction, X-ray diffraction and Raman spectroscopy analysis results are in accordance with STEM results providing the key attributes of spinel crystal structure of HEO NPs. XRD results also indicate the presence of lattice distortions in the synthesized HEO NPs. Raman spectroscopy modes A_1g, F_2g and A_1g* corresponding to divalent metal cation – oxygen (M²⁺- O) stretching vibrations indicates the evidence of spinel crystal structure fingerprints. X-ray photoelectron spectroscopy results provide the insightful understanding of chemical oxidation states of individual elements and their possible cation occupancy sites in the spinel-structured HEO NPs.

The remarkable versatility of the finite element method is used to model the three-dimensional discrete dislocation dynamics in heterogenous materials and structures. The present approach is based on the finite element strategy using advanced and flexible adaptative remeshing strategies for fine mesh resolution of complex curved dislocation loops. The numerical framework is capable of computing the stress fields and the driving forces acting on arbitrarily-shaped dislocations with a high degree of accuracy and with significant precision, as compared to novel theoretical elastic solutions for dislocation loops in the context of anisotropic hetero-elasticity theory.
The first part of the presentation continues beyond the theoretical contributions in the research area by treated dislocation loops in hetereostructures formed by elastically anisotropic crystals using the Stroh formalism combining with biperiodic Fourier-transform and dual variable and position techniques. The non-singular stress fields and driving forces are also used to investigate the comparison with the numerical finite element analysis of complex dislocation configurations. The fundamental Peach-Koehler force that governs the motion of the discrete dislocation loops is therefore captured by the numerical driving force calculation based on dissipative energy considerations. Because the numerical results show excellent agreement with analytical solutions in terms of non-singular stresses and forces, the finite element approach with adaptative remeshing techniques is used to explore more sophisticated microstructural situations, thus offering more realistic boundary-value description of dislocation loops in heterogenous materials and structures.

Solid-state thermionic structures made out of 3-5 layers of van der Waals heterostructures sandwiched between metallic electrodes have shown promising thermal to electrical energy conversion efficiencies theoretically. Here, I discuss our detailed theoretical studying of these structures using first-principles calculations combined with Green’s function method. We study the effect of the number of layers, the energy barrier, and the asymmetry of the contacts on the performance of transition metal dichalcogenide (TMD)-based thermionic converters. We show that the key is to make low-energy, low-resistance metallic contacts and we identify copper as the optimum metal to make ohmic low-resistance contact to MoSe2. Experimental demonstration of these structures involves a simple but mixed dimensional structure. I will next discuss the thermoelectric transport in mixed-phase 1T-2H MBE grown NbSe2 samples and will discuss the thermal, electrical and thermoelectric transport properties of these mixed phased samples.

Semiconducting two-dimensional (2D) materials possess intriguing optoelectronic properties deriving from a strong light-matter interaction and large exciton binding energies, which create opportunities for novel room-temperature optoelectronics such as excitonic computing with optical interconnects. It is therefore of great interest to engineer and modulate excitonic behaviors. The most widely used approaches to modulate excitons in semiconducting 2D materials are tuning the dielectric environment,[1] applying strain,[2] and applying external fields.[3] Here we describe a platform to modulate exciton emission in monolayer MoS₂ by integrating dielectric Coulomb engineering, local strain, and ferroelectric doping. Mixed-dimensional heterostructures consisting of monolayer MoS₂, P(VDF-TrFE) copolymer, and hBN were fabricated by transferring 1L-MoS₂ flakes on sub-μm P(VDF-TrFE) islands on hBN substrates. Raman and photoluminescence (PL) maps were correlated with atomic force microscopy topography to determine how local strain, dielectric contrast, and ferroelectric doping for the MoS₂ layer induced by P(VDF-TrFE) islands influence exciton emission.
Notable differences in Raman and PL spectra were observed between MoS₂ on flat hBN, thinner islands, and thicker islands. The Raman E mode is redshifted on thicker islands, consistent with tensile strain generated by the topography. In addition, the PL A exciton intensity is enhanced, which could be associated with a strain-induced funneling effect that would reduce the band gap. However, the MoS₂-A exciton emission energy is increased on P(VDF-TrFE) islands, which likely arises from the decrease in exciton binding energy due to the strong dielectric screening of the P(VDF-TrFE) (ε_r ≈ 10). Effects of dielectric environment and local strain on excitonic dynamics were further examined by analyzing the A and B exciton intensity ratio; a high A-to-B intensity ratio is a sign of low nonradiative recombination and is desirable for excitonic device applications. Regions of increased A/B ratio (up to 2x) correlate well with the P(VDF-TrFE) island locations. Moreover, the B intensity showed a negative correlation with height, suggesting that enhanced substrate screening combined with higher strain from the morphology enhances the intravalley scattering from the B exciton state to the A exciton state. The A and B exciton energy difference map demonstrates that, in contact with P(VDF-TrFE), the spin-orbit coupling-induced valence band splitting is reduced, which could lead to an increased intravalley scattering rate. Finally, the ferroelectric P(VDF-TrFE) islands were poled by applying a bias to the Si substrate with respect to conductive AFM tip. The MoS₂ A-to-B intensity ratio increases by a factor of two on negatively poled islands, whereas positive poling does not substantially change the intensity ratio. The correlation of Raman, PL, and topography maps revealed that the substrate dielectric screening, local strain, and ferroelectric doping provide a means to localized and modulate excitonic emission in monolayer MoS₂.
[1] Raja et al., Nat. Commun. 2017, 8, 15251.
[2] Castellanos-Gomez et al., Nano Lett. 2013, 13, 5361.
[3] Mak et al., Nat. Mater. 2012, 12, 207.

In this talk we will discuss a new approach for preparing 0D-2D quantum dot-in-matrix heterostructures. We combine colloidal nanocrystal synthesis, self-assembly, and solution phase epitaxial growth techniques to develop a general approach for preparing single dot thick atomically attached quantum dot (QD) superlattices. These materials exhibit high quality translational and crystallographic orientational order along with state-of-the-art uniformity in the attachment thickness. Colloidal synthesis of well-faceted hexagonal prism shaped core/shell QDs, followed by liquid subphase self-assembly and immobilization of superlattices on a substrate creates a highly ordered structure for subsequent attachment. Importantly the nanocrystals are arranged such that they share a mutual crystallographic alignment. To attach the nanocrystals, we use a solution phase method to epitaxially grow additional semiconductor material in the voids between the particles resulting in a QD-in-matrix structure. The photoluminescence emission of the QD-in-matrix structure retains characteristic 0D electronic confinement. Finally, annealing of the resulting structures removes inhomogeneities in the QD-QD inorganic bridges, which atomistic electronic structure calculations demonstrate would otherwise lead to Anderson-type localization. The piece-wise nature of this methodology allows one to independently tune the size and material of the QD core, shell, QD-QD distance, and the matrix material. These four parameters can be tuned to control many properties depending on the specific applications (e.g. degree of quantum confinement, quantum coupling, band alignments, etc.). Further we can transform the QD superlattice to additional materials as demonstrated herein with a CdSe/CdS to HgSe/HgS conversion.

Transition metal oxides often having a perovskite structure form a wide and technologically important class of compounds. In these systems, ferroelectric, ferromagnetic, ferroelastic, or even orbital and charge orderings can develop and eventually coexist. These orderings can be tuned by external electric, magnetic, or stress field, and the cross-couplings between them enable important multifunctional properties, such as piezoelectricity, magneto-electricity, or magneto-elasticity. Here, will present the effect of thickness on PrVO3 thin films where a "dead layer" is found, and I will particularly focus on the change on the structural and magnetic properties with different substrates.

Financial support from ANR and Region Normandie (INCOX project) are acknowledged.

It is possible to synthesize two-dimensional (2D), atomically thin metals using confinement heteroepitaxy (CHet), in which plasma treated epitaxial graphene (EG) is intercalated by a metal precursor such as Ga or In at near-atmospheric conditions. The resultant quasi-freestanding epitaxial graphene (QFEG)/2D metal heterostructure ideally provides a high-quality graphene substrate, enabling the scalable synthesis of vertical heterostructures with additional 2D materials, such as MoS₂, which are expected to exhibit extraordinary optical and sensing properties. However, this is limited by the surface quality of the QFEG which directly impacts the adlayer nucleation, grain size, and performance.

We demonstrate that the traditional route of CHet leads to particulates on the surface following metal intercalation that can heavily impact the ability to create heterostructures with TMDs or TI adlayers. Through the growth of monolayer EG with small regions of buffer layer (BL), we show that intercalation occurs through the BL regions while maintaining high surface quality QFEG with pristine 2D metal underneath. Through use of atomic force microscopy (AFM), Raman, and x-ray photospectroscopy (XPS), we demonstrate that the 2D metal forms over the entire surface, leaving behind an atomically smooth graphene surface for subsequent growth of high quality 2D materials on these heterostructures.

Two dimensional (2D) electron transport has been one of the most important topics in science and technology for decades. It was originally studied in 3D semiconductors and then continued in 2D van der Waals (vdW) crystals. In this talk, I will start with the large-scale processes for generating 2D crystalline semiconductor films and superlattices that could be used to fabricate atomically thin integrated circuits. Then we will discuss more recent directions, where we use these 2D materials to realize non-electronic 2D transport phenomena, for example, observed from phonons, photons, and mass. These new approaches could empower the development of 2D phononics, 2D photonics, and 2D mechanics.

We report the nanoscale characterization of the heterojunction in MoS₂-WS₂ hetero-monolayers by means of Tip Enhanced Raman Scattering (TERS). We demonstrate that a red-shifted PL in WS₂ adjacent to the MoS₂ core, observed in as-grown samples on sapphire substrates with confocal Raman/PL microscopy, is associated with a heterojunction boundary that shows significant alloying and a varying width from about 25 nm (pixel size limited) to few hundreds of nanometers even within the same crystal. In TERS spectra collected with resonant 638 nm excitation, we observed significant increase in the intensity of the 200 cm^-1 and 215 cm^-1 peaks over the heterojunction boundary as well as a shift of the 433 cm^-1 peak in the WS₂ shell to 444 cm^-1. TERS signal of the same area collected with 785 nm laser showed no significant peaks in the 140-240 cm^-1 range, while the 444 cm^-1 peak was preserved, being slightly shifted to 442 cm^-1. The spectral position of this latter Raman peak as well as the presence of the resonant peaks in 140-240 cm^-1 range proved to be related to the degree of alloying in Mo_xW_(1-x)S₂ compounds. Based on the specifics of the gap-mode TERS response, we discuss the in-plane vs. out-of-plane nature of the composition-specific Raman peaks. TERS imaging proved to be a valuable characterization technique enabling cross-correlation of the nanoscale structural composition of 2D heterostructures with useful observables like the surface potential, topography and other physical properties probed by scanning probe microscopy.

Broadband photodetection and imaging have been of significant interest because different information is obtained depending on the wavelength. Especially, short-wave infrared (SWIR) has definite advantages such as a high visibility in adverse weathers and at night. Moreover, compared to mid- or far-wave infrared for thermal imaging, SWIR allows a lower dark current at room temperature due to a larger photon energy. On the other hand, visible light (VIS) provides color information, which can utilize identifying of traffic lights. Therefore, demand for broadband photodetection ranging from VIS to SWIR is increasing in many further applications, including autonomous vehicles. For broadband detection, photodetector of various structures has been proposed such as photodiode and field-effect phototransistor. Photodiode such as MoS₂/BP heterojunction shows high photoresponse without photocurrent gain, whereas field-effect phototransistor such as Si QDs/graphene FET shows high responsivity with slow photoresponse speed due to trapping photogenerated carriers.
In this paper, we have proposed and investigated n-MoS₂/p-Ge/n-Ge van der Waals heterojunction phototransistor for broadband photodetection with photocurrent gain and fast photoresponse speed. Transition metal dichalcogenides (TMDs) can easily form van der Waals heterojunction (vdWH) despite lattice mismatch. To achieve broadband detection, heterostructure was composed of large-bandgap MoS₂ and small-bandgap Ge. Moreover, n-MoS₂/p-Ge/n-Ge phototransistor forms high potential barrier in the conduction band of heterojunction, which blocks the transport of the electrons from n-MoS₂ to n-Ge. With illumination, photogenrated holes are accumulated in p-Ge, which lower potential barrier in the conduction band and thus a large number of electrons pass through the heterojunction. Hence, this device exhibits high photocurrent in spite of weak illumination.
Photoresponse characteristics of fabricated MoS₂/Ge vdWH phototransistor were measured with floating base. In I_C-V_CE curves under VIS (466 nm) and SWIR (1550 nm) illuminations of 10 mW/cm², broadband photodetection was clearly observed, caused by VIS detection in large-bandgap MoS₂ and SWIR detection of small-bandgap Ge. From measured results, responsivity (R), external quantum efficiency (EQE), and detectivity (D^*) were calculated. As V_CE increases, R and EQE linearly increase regardless of the wavelength because higher V_CE lower potential barrier in heterojunction. Under VIS (SWIR) illumination, R and EQE are the maximum measured values of ~7.4 (~4.7) A/W and ~1969 (~376) % at V_CE = 4 V, respectively. MoS₂/Ge vdWH phototransistor shows high photocurrent gain regardless of wavelength because photogenerated holes are accumulated in base and then potential barrier is lowered. Despite higher R with increasing V_CE, D^* is constant with value of ~4×10⁷ (~2×10⁷) Jones at 466 (1550) nm illumination due to high noise caused by the high dark current.
Transient characteristics were measured under VIS and SWIR laser modulations with frequency of 20 Hz. From these results, rise times (t_rise) and fall times (t_fall) were extracted and shown fast response speed with 202.3 (88.1) μs and 214.9 (232.8) μs of t_rise (t_fall), respectively. This results show that the fabricated device has a fast enough photoresponse speed to be used as VIS/SWIR broadband image sensor.
Demand of VIS/SWIR broadband image sensors is increasing, which require photocurrent gain and fast photoresponse speed. In this work, we propose and fabricate n-MoS₂/p-Ge/n-Ge vdWH phototransistor. The proposed device absorbs VIS in MoS₂ with large bandgap and SWIR in Ge with small bandgap, enabling broadband photodetection. Moreover, vdWH phototransistor not only have photocurrent gain, but also have fast photoresponse speed at the same time. We will confirm the possibility with the broadband image sensor with additional imaging measurements later.

Van der Waals (vdW) solids can be engineered with atomically-precise vertical composition through the assembly of layered 2D materials (2DMs). To date, most structures that have been constructed to study their interesting phenomena have been produced using an artisanal method of assembling micromechanically-exfoliated flakes. However, further engineering and application of vdW solids requires a scalable and rapid production method to precisely design and control composition and structure over all three spatial dimensions (x, y, z) and interlayer rotation (θ). Here, we demonstrate such an approach, Robotic 4D Pixel Assembly, for rapidly manufacturing designer vdW solids with unprecedented speed, area, patternability, and angle control. We utilize the robotic assembly of prepatterned pixels made from atomically-thin 2DM components. Wafer-scale 2DM films are grown, patterned through a clean, contact-free process, and assembled together with engineered adhesive stamps actuated by a high vacuum robot. Our technique led to the fabrication of vdW solids of up to 80 individual layers, consisting of 100 μm² to 1 mm² areas with pre-designed patterned shapes, laterally/vertically programmed composition, and controlled interlayer angle. This enables us to create efficient optical spectroscopy assays of vdW solids, where we can study the layer dependent evolution of optical properties. Furthermore, our approach allows for the fabrication of twisted N-layer assemblies, where we observe atomic reconstruction of twisted 4-layer WS₂ at unexpectedly high interlayer twist angles of ≥4°. Our vdW solids manufacturing enables rapid construction of atomically-resolved quantum materials and will help realize the full potential of vdW heterostructures as a platform for novel physics and advanced electronic technologies.

Complex oxides are of great interest for their rich variety of chemical and physical properties including magnetism, ferroelectricity, multiferroicity, catalytic behavior and superconductivity. Up to date, the preparation of crystalline complex oxide thin films has been mainly limited on substrates that can stand high temperature thermal treatments and on single crystal substrates when epitaxial growth is pursued.
These requirements dramatically limit their applicability excluding the possibility to prepare many artificial multilayered architectures to investigate emergent phenomena that arise in thin films and at their interfaces, as well as fabrication of flexible devices.
In the last few years, it has been proved successful the use of water-soluble Sr₃Al₂O₆ (SAO) as sacrificial layer to detach perovskite complex oxides from the growing substrate and freely manipulate them. In these cases, in-situ and high vacuum physical deposition techniques such as Pulsed Layer Deposition (PLD) and Molecular Beam Epitaxy (MBE) have been used.

Herein, our goal is to achieve this SAO sacrificial layer for freestanding complex oxides through the use of cost-effective chemical deposition techniques with atomic precision such as Chemical Solution Deposition (CSD) and Atomic Layer Deposition (ALD).
First, we have developed a solution-based procedure to prepare high quality textured SAO films. The relation between precursor chemistry, thermal decomposition and epitaxial growth has been thoroughly investigated by means of X-Ray Diffraction, Atomic Force Microscopy and Transmission Electron Microscopy, between others. [1]
Then, we have investigated the viability of SAO to transfer a wide variety of oxide membranes ranging from binary oxides to ternary oxides (ferrites, manganites). These oxides have been deposited on SAO by different deposition techniques (CSD, ALD and PLD), and the effect of these deposition techniques on the oxide membrane quality has been compared. The influence of air exposure on SAO degradation to subsequently transfer epitaxy has been carefully studied by means of Reflection High-Energy Electron Diffraction (RHEED) and identified an effective approach to overcome it. Finally, we studied the transfer of oxide membranes to several flexible and rigid supports (PET, PDMS, silicon wafer, …). As a case example, we have studied the fabrication of CoFe₂O₄ membranes. The changes in epitaxy and magnetic properties throughout the transfer process are monitored by means of XRD and SQUID magnetometry obtaining crystalline and magnetic CoFe₂O₄ membranes. [2]
In short, this work introduces a chemical-based route to prepare freestanding complex oxides tackling the challenges related to this procedure. These results prove that chemical deposition could become an alternative to high vacuum physical deposition techniques, providing simplicity and cost saving to the process, and therefore making the freestanding complex oxides a step closer to large-scale production.

[1] P. Sallés, M.Coll et al., Adv Mater Interfaces, 2021, 8, 2001643,
[2] P. Sallés, F. Sánchez, M Coll et al., in preparation.

Rare-earth iron garnet thin films have advanced the development of on-chip non-reciprocal photonic and prototype spintronic devices due to their large Faraday rotations and spin diffusion lengths, respectively. However, non-epitaxial garnets deposited using pulsed laser deposition (PLD) or sputtering require an annealing temperature of 800°C or greater, and epitaxial garnet films are limited to lattice-matched substrates that are not foundry compatible. Here, a modified rapid thermal annealing process enabled mechanical exfoliation of high-gyrotropy cerium-doped terbium iron garnet (Ce_0.25Tb_2.75Fe_4.75O₁₂) from a silicon substrate without the need for an intermediary graphene layer, such as used in remote-epitaxy. Exfoliation is enabled by vacancy diffusion that followed a Nabarro-Herring model where strain was varied using different thicknesses for the garnet films. An increase in the tension between the film and the substrate resulted in a diffusion of vacancies away from the substrate. The diffusivities calculated by fitting the model to the strain rate-stress data identify iron and terbium as the rate-determining lattice diffusants. Furthermore, cross-section energy dispersive X-ray spectroscopy shows an accumulation of cations to form an amorphous Fe-Tb-Si-O interlayer at the substrate, leaving behind vacancies that create an exfoliation plane ~30nm into the film. This distance is comparable to the cation diffusion length, verifying the model. Magnetic and optical measurements from the exfoliated nanosheets reveal a saturation magnetization of 18 emu cc^-1 and a Faraday rotation of -2900°cm^-1 at 1550 nm, both of which are comparable to bulk values. Exfoliation through controlled creep-diffusion will open non-traditional pathways for incorporating garnets with other crystalline wafer platforms and protect devices from the high-temperature processes often used in crystallizing garnet films.

Crystallization from an amorphous precursor via solid phase epitaxy (SPE) enables a wide range of opportunities in the formation of oxide materials in new geometries. The use of low deposition temperatures to form the amorphous precursor reduces diffusion and phase separation during deposition and thus allows the formation of chemically and structurally uniform starting materials. Kinetic phenomena arising during crystallization of oxide materials in the form of 3D nanostructures via SPE pose challenging synthesis and characterization problems. A key challenge is to achieve nanoscale control of epitaxial crystallization. Selecting the locations of the initial step of crystallization and simultaneously maintaining a low rate of nucleation away from epitaxial interfaces are particularly important. In order to probe the propagation of the crystallization transformation, the location of crystallization sites in a study of SrTiO₃ (STO) SPE were set by forming micron-scale Si₃N₄ (SN) features on SrTiO₃ 001 single-crystal substrates using optical lithography. Amorphous STO was deposited on the patterned substrates at room temperature using radio-frequency sputtering and crystallized by heating to temperatures at which crystallization occurred from seeds while the rate of homogeneous nucleation remained low. A further challenge is to understand the microscopic mechanism that leads to crystallization at amorphous/crystal interface in nanoscale complex geometries. An in situ synchrotron hard x-ray nanodiffraction instrument at the Advanced Photon Source enabled this process to be studied using nanobeam diffraction and reflectivity probes during amorphous deposition and epitaxial crystallization. The results indicated that amorphous STO selectively crystallized on STO seed and laterally crystallized over the SN patterns before encountering separately nucleated crystals. Three-dimensional SPE of STO provides opportunities to create a wide range of other perovskite oxides in nanoscale geometries. The results will be discussed in terms of the competition between crystal/amorphous interface motion and homogeneous nucleation, the stress-induced from the density difference between the amorphous and the crystalline films, and the consequences of small deviations from the desired STO stoichiometry.

Semiconductor heterostructures have been used extensively in the development of electronic and optoelectronic devices such as transistors, lasers, LEDs, and photodetectors. Traditionally, these composite materials are synthesized by heteroepitaxy, where films of different compositions are grown on top of one another through various atomic deposition techniques. This approach is well established in micro and optoelectronics, where it provides a powerful tool to shape the energy band profile in order to promote and enhance desired device functionalities.
Here we report a new method, based on strain engineering, for the controlled introduction of variations in bandgap energy with lateral position in thin films. External stress is applied on a semiconductor nanomembrane stacked with an array of dielectric pillars, in order to create a non-uniform strain distribution commensurate with the sample thickness variations (with the largest strain accumulating in the thinnest regions between the pillars and vice versa). In typical group-IV and III-V semiconductors, in-plane biaxial tensile strain has the effect of decreasing the fundamental energy bandgap, while at the same time lifting the degeneracy of the heavy-hole and light-hole valence-band maxima. The end result is therefore a controlled bandgap modulation – i.e., the formation of a sample that effectively behaves like a lateral (in-plane) heterostructure, but consisting of a single semiconductor material with uniform chemical composition.
Specifically, we employ 90-nm-thick Ge nanomembranes coated with a square-periodic array of cylindrical amorphous-Si pillars having micron-scale lateral dimensions and 300-nm height. The nanomembranes are transferred and bonded on a flexible film of polyimide, which is then mounted on a gas pressure cell that allows introducing biaxial tensile strain in the Ge crystal in a highly controllable fashion. The resulting strain profiles are mapped using Bragg diffraction with a hard x-ray probe featuring nanoscale spatial resolution. A clear lateral strain modulation is observed in the measurements, with the strain going through a well-defined minimum at the location of each amorphous-Si pillar. The strain difference between coated and uncoated nanomembrane regions is also found to increase with increasing gas pressure, in agreement with finite-element simulations. The measured strain maps are finally used to compute the Ge-nanomembrane energy band lineup, showing the formation of pronounced energy barriers in both conduction and valence bands.
Compared with traditional heterostructures grown by epitaxial techniques, these strain-engineered samples are not limited in the choice of compatible materials by any restriction imposed by lattice-matching requirements. Furthermore, their energy band lineups can be patterned in nearly arbitrary shapes using nanolithography to control the thickness profile, and can be tuned actively by varying the applied stress. As a result, these structures are attractive for a wide range of device applications (including lasers, LEDs, solar cells, and thermoelectrics) that require complex heterostructure lineups with multiple bandgap energies.

The fast development of nanotechnologies in the past decades desires various nanomaterials that span wide ranges of sizes, geometry, materials choice, crystal structures, and elemental distributions. However, the availability of these nanomaterials is usually leashed by the ability of specific nanofabrication methods in at least one of these aspects.

First applied for polymers and BMGs, nanomolding refers to a top-down fabrication method by which a formable or moldable material is shaped using a mold of nanoscale dimensions. Recently, thermomechanical nanomolding (TMNM) has been developed for general crystalline materials, indicating potentially a very versatile method for large scale nanofabrication.

In this work, with carefully designed scaling experiments, we revealed the principle behind TMNM to be a diffusional atomic transport that happens at the very interface between material and mold. By controlling processing temperature and length scale, we are able to tune TMNM between this interface-diffusion principle and a dislocation-slip dominated regime. Such a transiting mechanism results in a highly versatile toolbox, enabling fully controlled nanofabrication over sizes, geometry, materials choice, and crystal structures. Specifically, the element dependence of diffusivity allows controlled growth down to single elements/components of the feedstock, indicating controllable elemental distributions. And by utilizing TMNM, we have successfully demonstrated the fabrication of heterostructure nanowires with fully controlled elemental layer sequences.

We will discuss the transiting mechanism behind TMNM and the versatile toolbox by utilizing this mechanism including controlled fabrication of heterostructures. The possibility of forming new phases through kinetical control of elements in TMNM will also be discussed.

Two-dimensional (2D) fused aromatic networks (FANs) have a planar structure, in which flat building blocks are horizontally connected to form optimized, layered π-π stacking. Three-dimensional (3D) FAN structures, in which building blocks are divergently linked in the space, provide the maximum accessible internal surface area with permanent porosity. As sorbent materials, the confined layered stacking pattern of FANs restricts segmental motion, and the available surface area is partially blocked by adjacent layers, resulting in weak uptake capacity and kinetics. To understand the relationship between structure and performance, here, a new type of pyrazine-linked FAN structure, with vertically standing building blocks connected along the 2D direction, was designed and synthesized to minimize layer-to-layer interactions and to maximize segmental freedom. In comparison studies with flat 2D and porous 3D FANs, the vertically standing FAN (V2D-Pz) displayed intermediate performance for gas storage and separation. For example, the results for the adsorption and separation of CH₄/N₂ fell in between those of the flat 2D and porous 3D FANs. However, the standing V2D-Pz had a more structurally accessible adsorption area and higher segmental freedom, and it exhibited the highest iodine uptake capacity (~4.2 g g^-1) and capture rate (1.51 mmol g^-1 h^-1).

Grain-boundary engineering is essential to unleash the potential of materials. While a number of methods have been used to tune up grain-boundary conditions such as hammering, plasma-assisted sintering, deposition, etc, it still remains challenging to achieve precise grain boundary engineering at the nanoscale because these methods rely almost exclusively on top-down approaches. Meanwhile, recent theoretical studies have made a striking advance in the understanding of structure-properties relationships over nanoscale grain-boundary conditions and novel materials’ design strategies, thus, a methodology to make a tailored grain-boundary condition in real experiments is wanted for experimental demonstration. Herein, we introduce a new grain-boundary engineering method, “the nanocrystal coining method”. With this method, various grain-boundary factors such as grain-boundary shape and size are readily tuned at least one-nanometer precision. The produced materials possess a centimeter-scale size and freestanding nature, which is ideal for many applications. Other advantages include a wide range of available components and no restriction of substrates. In this presentation, we will discuss the nanocrystal coining method in the greater details
The nanocrystal coining method uses chemically synthesized colloidal metal nanocrystals as the starting materials. We treated the nanocrystals using a surface treatment, thorough washing and drying process, and a pressure process with a piston-cylinder pressure die. The resulting materials (which we call ’’nanocrystal coins’’) showed several strikingly improved properties owing to the nanoscale grain-boundary condition and the high material density. The nanocrystal coins successfully emerged metallic properties in the appearance and conductivity while the crystal domains of the original nanocrystal building blocks remained unchanged. In addition, nanoindentation measurements confirmed the superior mechanical hardness of the nanocrystal coins due to the Hall-Petch effect. Taking further advantage of this method, we created the first example of a single-component bulk metallic glass from amorphous palladium nanoparticles. The MD simulation supported the mechanisms of the nanocrystal coining method including preservation of grain-boundary condition, low-pressure sintering, and densification under pressure. Our methods will help researchers to finely iterate structure-properties relationships whose functionality crucially depends on the crystal-domain configuration, such as superhard materials, thermoelectric generators, and functional electrodes.

Three-dimensional (3D) heterostructures with incommensurate arrangements of well-defined building blocks are created using an unconventional synthetic approach comprising of mechanically facilitated disassembly and reordering of layered transition-metal dichalcogenides, MX₂ [1,2], and non-layered rare-earth metal monochalcogenides, REX [2], where M = Ta or Nb, RE = Sm or La, and X = S or Se. The discovered solid-state processes are driven by mechanochemical transformations directed by quantum interaction between chemically and structurally dissimilar solids toward atomic-scale ordering.

Remarkably, in the case of a pair of layered MoS₂ and HfS₂ compounds[1], the experimentally observed 3D-heterostructuring is energetically favorable over the formation of homogeneous multi-principle element dichalcogenides that had been observed in related dichalcogenide systems including MoS₂, WS₂ , and TaS₂ materials [3].

Density-functional theory calculations reveal critical details about charge transfer between structural elements of the explored heterostructures and validate experimental results. High-temperature annealing improves crystallinity of 3D-heterostructures without altering their internal atomic arrangements, which resembles respective behaviors of molecular solids and polymers.

Powder X-ray diffraction, scanning transmission electron microscopy, ⁷⁷Se solid-state nuclear magnetic resonance, optical spectroscopy and electronic transport measurements unambiguously confirm the course of mechanochemical reactions and the identities of the created materials. The resulting 3D-heterostructures show broad range of electron transport behaviors varying from metallic conductivity to indirect band gap semiconductivity.

Results of this joint experimental and theoretical study open new avenues for generating unexplored metal-dichalcogenide heteroassemblies with incommensurate structures and tunable physical properties.

References:
I. Z. Hlova et al. Nanoscale Adv. (2021), Ahead of Print.
O. Dolotko et al. Nat. Commun. (2020), 11(1), 3005
I. Z. Hlova et al. Chem. Commun. (2018), 54(89), 12574

Co-axial electrospinning was applied for the structuring of non-woven webs of TiO₂ nanofibers loaded with Ag, Au, and CuO nanoparticles (NPs). The composite layers were tested in an electrochromic half-cell assembly. A clear correlation between NP composition and electrochromic effect in the nanofibrous composite is observed: TiO₂ loaded with Ag reveals a black/brown color, Au shows a dark blue color, and CuO a dark green color. For electrochromic applications, the Au/TiO₂ layer is the most promising choice, with a remarkable color modulation time of 6 s, transmittance modulation of 40%, coloration efficiency of 20 cm²/C and an areal capacitance of 300 F/cm², and cyclic stability of over 1000 cycles. Electrochemical impedance spectra of the best performing sample, Au/TiO₂ fibers, was also analysed to understand the effects of additive NPs within the fibrous network. In this study, we offer an unexplored path for the rational design of TiO₂-based nanofibrous layers with unprecedented color switching and optical efficiency gained by the fibrous layer. We foresee co-axial electrospinning as an alternative nanofabrication technique applied in smart colored windows.

The past decade witnessed several developments on 2D graphene analogs, also known as Xenes, and several milestones have been achieved on material synthesis and their application prototypes¹. If the synthesis of new Xenes and their application prototypes are widely studied, piling up one Xene layer over another Xene layer to realize a Xene heterostructure to the date remains a challenging issue owing to interlayer mismatch, intermixing, etc.. Indeed, even if two different Xenes can be grown on the same substrate, composing heterostructure and superlattice geometry has remained quite puzzling. Making Xene heterostructure in this framework therefore will provide substantial technological throughputs for various applications, for instance nanoelectronics, optoelectronics, plasmonic and energy.

In this work, we first introduce the Xene heterostructure² concept and present two heterostructures based on two well-established configurations: silicene-on-Ag(111)³ and stanene-on-Ag(111)⁴. We will further demonstrate Xene superlattices taking advantage of underneath Xene layer as a suitable template for another reciprocal Xene layer. Furthermore, the insights into co-deposition techniques where our results show it is possible to achieve both the heterostructure sequence by engineering the growth kinetics will be discussed in detail with various in situ and ex situ probes. Moving beyond monolayer silicene-stanene configuration, we will also present layer-by-layer growth of multilayer 4x4 Silicene on the same reciprocal stanene-on-Ag(111) template. In the end, Density Functional Theory calculations were carried out taking experimental data as starting input models in order to have deeper insights into structural stability and electronic structure.

The work is performed within the ERC-COG 2017 Grant No. 772261 "XFab".

[1] L. Tao, E. Cinquanta, D. Chiappe, C. Grazianetti, M. Fanciulli, M. Dubey, A. Molle, D. Akinwande, Nat. Nanotechnol. 2015, 10, 227.

[2] D. S. Dhungana, C. Grazianetti, C. Martella, S. Achilli, G. Fratesi, A. Molle, Adv. Funct. Mater. 2021, 2102797. https://doi.org/10.1002/adfm.202102797

[3] P. Vogt, P. De Padova, C. Quaresima, J. Avila, E. Frantzeskakis, M. C. Asensio, A. Resta, B. Ealet, G. Le Lay, Phys. Rev. Lett. 2012, 108, 155501.

[4] J. Yuhara, Y. Fujii, K. Nishino, N. Isobe, M. Nakatake, L. Xian, A. Rubio, G. Le Lay, 2D Mater. 2018, 5, 025002.

Emerging field of 2D materials shifts nowadays towards preparation of metamaterials, combining efficiently functionality of 2D layers themselves with additional components such as nanocrystals, forming together multifunctional 2D material with complex properties². It has been shown that combination of 2D sheets with nanoparticles does not bring together just individual properties of both components³, but also affect behaviour of 2D sheets themselves⁴.
Functionalized 2D nanosheets can serve as basic bulding blocks for fabrication of larger structures targeting metamaterial with desired properties, for example via self-assembly, hence implementation of additional functionality directly into the 2D nanosheets is of high importance.
We present here an efficient approach of decorating 2D nanosheets in liquid phase with magnetic nanoparticles, based on functionalized iron oxide nanoparticles and high-aspect ratio sheets of insulating transparent synthetic clay, sodium fluorohectorite. Efficiency of the process is confirmed by several complementary characterization methods, including TGA, X-ray diffraction (SAXS/WAXS), spectroscopy (FTIR, UV-VIS). Particle layout on nanosheets is determined by microscopy techniques (SEM and AFM). Options of tuning particle density on nanosheets and particle geometrical arrangement in a view of possible applications will be discussed.

1. Chem Rev. 2015, 115(7): 2483–2531.
2. Chem. Rev. 2017, 117 (9): 6225-6331.
3. Nano Lett.2012, 122: 617-621.
4. Progress in Materials Science 2017, 90: 75–127.

Heterogeneous integration of transition-metal chalcogenides on III-V compound semiconductors provides development opportunities for a variety of new optoelectronic devices. In this talk, we present a novel method for heterogeneous integration of MoS₂ on III-V compound semiconductors. A thin film of MoS₂ was directly synthesized on a GaN-based epitaxial wafer through the MOCVD technique and fabricated as the TFT array. Subsequently, the MoS₂ TFT was monolithically integrated with micro-LED devices to produce an active matrix micro-LED display. This strategy represents a promising route to attain heterogeneous integration, which is essential for high-performance optoelectronic systems that can incorporate the established semiconductor technology and emerging 2D materials.

Perovskite nickelates undergo room temperature colossal electronic phase transitions which could modulate the electronic conductivity up to 8-10 orders of magnitude by hydrogen doping. This creates an unprecedented opportunity to fabricate 2D localization of electronic phases by atomic scale displacement of hydrogen in the lattice. One way to accomplish this is by locally storing hydrogen in metal gates such as palladium (Pd) that can be injected into the near surface perovskite lattice by an external electric field. Here we developed an asymmetric two-terminal device fabricated using SmNiO₃ (SNO) thin film which forms multiple heterostructures within each device. The two-terminal device, with one electrode comprising Pd and the other electrode gold (Au), enables a gradient in hydrogen doping which allows to create arbitrary electronic phases. We will describe the band structure physics of such novel electronic heterostructures that can form 2D electron puddles which can subsequently be distributed spatially by electric fields. The current-voltage characteristics show remarkable transitions in mechanisms depending on the local distribution of hydrogen proximal to the electrodes. We will discuss the response of such 2D-3D heterostructures to electric stimuli and the use in emerging neuromorphic computing architectures.

Despite significant progress in the spectral discrimination properties of photodetectors (PDs), most of the conventional technologies use optical filters which results in expensive circuitry as well as high incident energy loss. 2D/3D integration of an MoS₂/ZnO heterostructure-based PD has been demonstrated which exhibits a bias-dependent switchable spectral response, in a manner different from the traditional PDs. Low band gap MoS₂ thin film has been vertically stacked on top of a high band gap ZnO film that allows selective charge transport from each layer by modulating the applied bias, resulting in a fine discrimination between ultraviolet (UV) and near infrared (NIR) light. Under lower applied bias, a dominant photoresponse in the UV region by the ZnO film is observed, whereas an enhanced response in the NIR region is obtained at higher bias which is attributed to the generation of charge carriers in the MoS₂ film. The device predominantly exhibits UV photoresponse at 1.0 V with respnsivity of 4.56 AW^-1 (400 nm) and that in the NIR region at 8.0 V with responsivity of 6.04 x 10³ AW^-1 (900 nm). The excellent figures of merit of the device attest the high quality of the device. Thus, this approach paves a way for fabricating optical filter-free, low cost and energy-efficient imaging systems.

Heterostructures based on 2D materials have been found to harbor a plethora of intriguing physical phenomena whose presence is largely due to the availability of a vast 2D materials space and a plenty of degrees of freedom (e.g. twist angle, lattice mismatch) in structural design. Introducing 2D/3D heterostructures would further expand the design space for discovering new physics and developing novel devices. In this talk, I will present the design and development of heterostructures based on 2D materials and a 3D phase transition oxide. The relevant heterostructures in this talk include MoS₂/VO₂ and hBN/VO₂. In these heterostructures, the phase transition oxide imposes controllable mechanical and electronic boundary conditions to the 2D materials allowing the discovery of flexo-photovoltaic effect in MoS₂ and reconfigurable transport behavior in hBN/VO₂. Further, the weak 2D-3D interface coupling has been harnessed to control the phase transition dynamics of VO₂. The demonstration of functional behaviors at 2D/3D interface suggests the promises of using 2D/3D heterostructures for designing new electronic and optoelectronic devices.

Solid-state electronics are currently under development toward the technological paradigm shift from miniaturization to deformable form factor.[1] However, typical semiconductors, including Si, III-V, metal-oxides, etc., are not intrinsically soft because of mechanical properties of bulk crystalline with brittleness and rigidity. An innovative way to fabricate flexible electronics is to make the device overlayer away from the rigid substrate and to assemble the devices on the other soft substrate in a form of spatially separate chip arrays. However, to separate the overlayer from the mother substrate, high energy irradiation (e.g., laser lift-off) and the use of corrosive etchants should have been unavoidable before emergence of non-covalent epitaxy (i.e., van der Waals and remote epitaxy).[2] The remote epitaxy is to grow epitaxial overlayer remotely from wafer across a gap made of two-dimensional (2d) atomic layer.[3] The technique has no restriction to obtain in use of poly-domain graphene for growing single crystalline overlayer, which is distinct from the van der Waals epitaxy whose overlayer follows the surface crystal structure of 2d layer. In this talk, we focus to present and discuss how the remote epitaxy of micro- or nano-sized epitaxial architectures is applied to fabricate deformable and transferable devices. First, the epitaxy regime (i.e., covalent, remote, and van der Waals epitaxy) is discussed in terms of attracting force from wafer and graphene and role of graphene. We introduce the random nucleation-based remote epitaxy of ZnO and GaN micrometer-sized rod, which the spatially isolated geometry can be devised to fabricate the flexible light-emitting diodes (LEDs).[4] We also present the white LEDs fabricated by the remote epitaxy of GaN p–n junction microrods.[5] To increase the versatility of the remote epitaxy, it is necessary to control the position and size of rod structures. Thus, the methods of site-selective remote epitaxy are presented.[6, 7] The density-functional theory simulation and atomic-resolution transmission electron microscopic analysis results are also accompanied to discuss the mechanism of remote epitaxy.

[1] D. H. Kim et al., Adv. Mater. 22(19), 2108 (2010).
[2] K. Chung, C.-H. Lee, G.-C. Yi, Science 330(6004) 655 (2010).
[3] Y. Kim, S. S. Cruz, K. Lee et al., Nature 544, 340 (2017).
[4] J. Jeong, Q. Wang, J. Cha et al., Sci. Adv. 6(23) eaaz5180 (2020).
[5] J. Jeong et al., Nano Energy 86, 106075 (2021).
[6] J. Jeong et al., ACS Appl. Nano Mater. 3(9), 8920 (2020).
[7] D. K. Jin et al., APL Mater. 9(5), 051102 (2021).

Complex-oxide materials exhibit a plethora of physical properties desirable for next-generation electronics, photonics, and quantum devices. These properties may be present in bulk form or at a heterointerface, and may be created or enhanced by application of strain. However, to date, only epitaxial methods have been used to create various heterostructures with varying strain states. Unfortunately, epitaxial methods have strict limitations, requiring closely lattice mismatched and similar crystalline structure, which prevent unrestricted manipulation, integration, and utilization of these materials. Thus, the design space for heteroepitaxial systems are severely limited. Additionally, the range of strain that can be imparted by epitaxy is fixed by pseudomorphic conditions. Finally, epitaxial growth conditions, which typically occurs at high temperatures, preclude integration of materials that are less stable in such environments. This has been a consistent bottleneck preventing universal integration of dissimilar materials to date.

To alleviate these restrictions, we present methods to produce freestanding complex-oxide membranes allowing artificial creation of complex-oxide heterostructures with vastly different crystalline structure, lattice, and orientation, such as perovskite, spinel, and garnet films. The first lift-off method, remote epitaxy, allows epitaxial seeding of the substrate through monolayers of graphene, which can then be easily exfoliated due to the van der Waals nature of graphene. However, some complex-oxide materials, such as piezoelectric PMN-PT, are grown by sputtering, which damages the graphene due to the harsh plasma ambient. For these materials, we found that by choosing a substrate with a specific lattice misfit, the epitaxial layer can be precisely separated from the substrate using a high stress Ni layer.

We also demonstrate artificial heterostructures by stacking exfoliated complex-oxide membranes manually, observing the mechanical and physical interaction between the stacked membranes. We hope such demonstration of artificial 3D-3D and 2D-3D heterostructures will allow to significantly expand the possible permutations of materials and allow experimental realization of structures not possible by epitaxial means alone.

Wide-bandgap semiconductors, especially group-III nitrides (III-Ns), have excellent performance to be applied into optoelectronic and electronic devices in full-color range. In industry, as the most used epitaxial substrate, sapphire however, has large thermal and crystal mismatch with these nitrides and induces large compressive strains and density of dislocations, seriously troubling the device performance.
Recently, with the assistance of two-dimensional (2D) materials, such as graphene, remote epitaxy (or called quasi-van der Waals epitaxy) has given new choice to semiconductor films epitaxy. The weak van der Waals force in the nitride/graphene/sapphire sandwich structure alleviates the lattice and thermal mismatch, tunes nucleation behavior, reduces growth time, and/or achieves damage-free transfer for high-power and flexible devices. But still, owing to the lack of atomic resolution characterizations, the role of graphene acts in this process is still not clear. Meanwhile, the 2D materials transfer process which formal study mostly focused on can hardly be used into industry. Herein, we share some study about the atomic structure of nitride/graphene/sapphire interface and the mechanism of remote epitaxy, in which the graphene/sapphire substrates are 10*2-inch batch synthesized by chemical vaper deposition (CVD) method, offering opportunity for the usage in industry.
Firstly, we study the surface of the graphene covered sapphire. Instead of van der Waals interaction, the grown graphene is C-O-Al bonded with sapphire unexpectedly ^[1]. Compared with transferred sample, the grown graphene is stronger pasted on sapphire with larger lateral force, and the structure relaxation behavior of sapphire near the surface is suppressed compared with that of the bare surface.
Upon the graphene/sapphire substrate, we grow GaN film with metal organic CVD (MOCVD) method. After this process, the sapphire surface transforms into a high Al-O ending surface, significantly different from the case before MOCVD. We also find that the whole nitride film achieves low compressive strain as well as low density of dislocations. Based on the high-quality nitride films, we further fabricate high In composition InGaN alloy. The strain relaxed nitride film creates suitable environment for up to 32% In element doping. The as-fabricated light emitting device (LED) shows a 563 nm electroluminescence emission, corresponding to yellow light.
[1] Dou et al., Nature Communications 10, 5013 (2019)

Remote epitaxy is novel synthesis technique that allows the fabrication of ultrathin, flexible, and highly unique heterostructures that cannot be created by other methods. The technique was recently applied to construct freestanding, complex oxide membranes [1]. By inserting graphene between the film and substrate during thin film growth, the van der Waals interactions between graphene and other materials are strong enough to enable epitaxial growth but weak enough to allow the rapid release the thin film. The technique may eventually be exploited to create novel, single crystal heterostructures, as each film can be “stacked” atop another, regardless of degradation due to misfit strain or incompatibilities due to high temperature reactions. However, details regarding how remote epitaxy actually occurs remains unknown as this requires an in situ investigation of the remote epitaxy process. Here we conduct in situ synchrotron X-ray measurements during growth by molecular beam epitaxy (MBE) to observe epitaxial synthesis through graphene layers. COBRA (COherent Bragg Rod Analysis) is used to reconstruct the electron density profiles of the epitaxial system after the growth of each monolayer. Subtle changes of the graphene layers during heating and in different oxygen pressures were captured, providing crucial details in understanding the remote epitaxy technique.

[1] Hyun S. Kum et al., Nature 578, 75 (2020).

Work supported by the Department of Energy, Office of Science, Basic Energy Sciences under contract no. DE-AC02-06CH11357.

Van der Waals (vdW) epitaxy of 3D device structures on 2D layers is particularly interesting for III-nitrides and has attracted intensive investigation. Dislocations and defects seen in typical nitride 3D epitaxy could be avoided when grown on a 2D layer, because strain due to large lattice and thermal mismatches relaxes elastically during 3D growth. The technique is also very convenient for heterogenous integration. With simple sticky holders (tapes, PDMS, metallic films…), the III-N heterostructures can be mechanically detached over a large area from its substrates and the membrane transferred to arbitrary carriers, at minimum of time and low cost. Hexagonal boron nitride (hBN) is an excellent choice of 2-D material. It exhibits good stability at high temperature, excellent compatibility with III-nitride growth conditions and growth systems (MOCVD, MBE, CVD…) and can be grown on different types of substrates, with sapphire our substrate of choice.
We have demonstrated a range of devices using 2D/3D vdW epitaxy: LEDs, solar cells, sensors and HEMTs. In this talk we overview our results in epitaxy and materials characterization, device fabrication, mechanical lift-off and transfer, and pre- and post-transfer device characterization. Different mechanical liftoff and transfer approaches will be compared in term of material quality, crack generation and flexibility. Going beyond wafer-scale epitaxy, we will also present selective area vdW epitaxy and self-lift off and transfer techniques that can further simplify and lower the cost of heterogenous integration because of innovative epitaxy and process fabrication that use commercial systems.

The unique electrical and thermal properties of graphene stimulated the search for other two-dimensional (2d) atomic crystals with unusual properties. A number of 2D materials belong to the family of transition metal chalcogenides that contains weak van der Waals bonding between structural units. Until recently, most research utilized dichalcogenides such as MoS2, TaS2 and others, distinguished by 2D layers. Another class of transition metal chalcogenides – the trichalcogenides – has quasi-1D crystalline structures. In this presentation, I review our results, which show that the mechanical and chemical exfoliation approaches developed for 2D materials can be extended to a relevant class of quasi-1D van der Waals materials such as TaSe3, ZrTe3 and others. Electrical measurements established that the bundles of quasi-1D atomic chains have a breakdown current density approaching 100 MA/cm2 [1, 2]. The atomic chain single crystal nature of TaSe3 results in low surface roughness and the absence of grain boundaries. These features potentially can enable the downscaling of these wires to lateral dimensions in the few-nm range. The polymer composites with low loadings of quasi-1D TaSe3 fillers reveal excellent electromagnetic interference shielding in the X-band GHz and extremely high frequency sub-THz frequency ranges, while remaining DC electrically insulating [3, 4]. The unique electromagnetic shielding characteristics of these films are attributed to effective coupling of the electromagnetic waves to the high-aspect-ratio electrically conductive TaSe3 atomic-thread bundles even when the filler concentration is below the electrical percolation threshold. These novel films are promising for high-frequency communication technologies, which require electromagnetic shielding films that are flexible, lightweight, corrosion resistant, inexpensive, and electrically insulating.

This work was supported, in part, by the NSF DMREF program via a project DMR-1921958 Collaborative Research: Data Driven Discovery of Synthesis Pathways and Distinguishing Electronic Phenomena of 1D van der Waals Bonded Solids. The author acknowledges the Vannevar Bush Faculty Fellowship (VBFF) with the funding for the project One-Dimensional Quantum Materials.

[1] M.A. Stolyarov, et al., Nanoscale, 8, 15774 (2016); [2] A. Geremew, et al., IEEE Electron Device Lett., 39, 735 (2018); [3] Z. Barani, et al., Adv. Mater., 33, 2007286 (2021); [4] Z. Barani, et al., ACS Appl. Mater. Interfaces, 13, 21527 (2021).

Integration of group III nitrides and two-dimensional buffer layers, such as graphene, has been considered as a new material system for the next-generation optoelectronic devices. However, the growth of high-quality semiconductor membranes on the slippery van der Waals surface remains challenging. Harsh metalorganic vapor phase epitaxy (MOVPE) environment and graphene transfer technique have a great impact on the successful realization of remote epitaxy.

In this work, one-step and multi-step MOVPE growth approaches were realized to fabricate GaN layers on GaN/sapphire templates covered with graphene. While the one-step approach led to the disappearance of graphene Raman fingerprints, the multi-step growth protocol resulted in the successful formation of GaN films on GaN/sapphire templates. The quality of grown layers was investigated using atomic force microscopy, X-Ray diffraction and photoluminescence techniques. Transmission electron microscopy analysis revealed a clean GaN/graphene/GaN interface.

The seeding effect of the substrate was previously shown to take place for up to two layers of graphene. The use of bilayer graphene could be favorable as it provides an aperture-free interlayer. To investigate the effect of the number of graphene layers and their transfer technique, seeds of GaN and InN were formed. Single transfer of bilayer graphene proved to be better suited for the realization of remote epitaxy rather than double transfer of monolayer.

Van der Waals (vdW) epitaxy has been studied as a promising materials preparation method which can overcome materials compatibility. Atomically thin two-dimensional (2D) materials serve as substrates of other low-dimensional and three-dimensional (3D) materials growth to provide surface dangling bonds-free surface, a prerequisite of vdW epitaxy. However, direct utilization of a 2D material as a substrate brings the issue of poor nucleation efficiency due to small number of nucleation sites where dangling bonds used to be. The nucleation efficiency issue has been resolved by dipole engineering method, which was developed by the presenter’s team. The dipole engineering technique is based on increase in surface energy of a 2D material by transfer of the 2D material on a bulk or a thin film on which polarization is formed along the out-of-plane direction. To induce the required out-of-plane dipole moment, applying electrical bias or transfer of a 2D material onto a ferroelectric material has been utilized. However, the demonstrated techniques of dipole engineering have not been suitable for recycling the substrates due to changes of the substrates’ characteristics under vdW epitaxy conditions. In the presentation, we will discuss a robust and recyclable dipole engineering based on refractory metallic-dielectric multilayers in which inherent out-of-plane dipole moments are formed. We transferred monolayer graphene onto the multilayer substrate. Then, crystalline germanium growth via chemical vapor deposition was successfully demonstrated.

The promise of remote epitaxy, in which epitaxial registry of a film can be maintained through two-dimensional layers, has emerged from an advanced understanding of the interactions between two dimensional materials and bulk semiconductors.[1] A promising application of this technology is to facilitate substrate reuse in epitaxial III-V systems, which could significantly reduce material cost and improve commercial viability of this high-efficiency photovoltaic technology. In this process the weak van der Waal bonds between the 2D layer and the III-V substrate make separation of epitaxially-grown layers simple and leaves a smooth substrate that can be reused for cost reduction.

Here we present a study of growth of GaAs on AlGaAs through a low dimensional carbon interlayer via hydride vapor phase epitaxy (HVPE). We Investigate the effect of various GaAs nucleation conditions such as V/III ratio, temperature and growth rate on film roughness, crystallinity, and degree of epitaxial alignment. We demonstrate growth of single crystal GaAs layer on top of the low dimensional carbon that follow the crystal orientation of the substrate. Surface roughness measured via AFM found that lower V/III ratios result in smoother III-V films grown on the low dimensional carbon, with a lowest RMS of ~60 nm obtained at a V/III ratio of 5. Growth temperatures are optimized at 650 °C, as temperatures below 550 °C show non-crystalline growth and higher temperature samples display very rough surfaces.

Additionally, we investigate HVPE growth on various forms of low-dimensional carbon including transferred graphene, CVD-grown graphene, and MOCVD-growth amorphous carbon and find the latter to be most well suited for III-V growth as this method results in cleaner interface and potential for direct, large-area coverage. We also identify techniques to mitigate surface degradation in carbon layers as a function of both exposure time and growth environment by analyzing surface changes via x-ray reflectivity. We show growth of heteroepitaxial III-V solar cells on carbon interlayers and preliminary cell results, which are currently limited to 4% efficiency under one sun.

[1] Y. Kim, S. Cruz, K. Lee, B. Alawode, C. Choi, Y. Song, J. Johnsen et al. Remote epitaxy through graphene enables two-dimensional material-based layer transfer. Nature 544 340-343 (2017).

Our group has recently optimized a reproducible and controlled growth process of a monolayer graphene on silicon carbide (SiC, (0001)) at low Argon (Ar) pressure (10mbar). We highlight that such process is compatible with electronic applications hence no transfer is needed. The graphene samples obtained using this process have a mobility up to 2000 cm2V-1 s -1, at room temperature, which corresponds to the state of the art on similar substrates. One way to enhance the graphene’s mobility is to reduce SiC steps density. These steps with different surface energies have different step velocities and therefore the steps are preferable to be bunched together aiming to minimize the total surface energy during the thermal treatment. This step bunching often occurs during epitaxial growth of graphene on SiC. The height and width of the steps depend on various factors, such as, polytype of SiC, miscut angle and growth conditions. First of all, the miscut angles in our SiC substrates have been measured using a combined optical alignment and X-ray diffraction method. This method, which is unusual for SiC substrates, allowed us to reveal cut-off angles inferior to 0.07° for our SiC wafers. This result led us to not take into account the effect of the miscut angle in our process and mainley foccus on the growth parameters. In this study, we have investigated both the etching of 4H-SiC (0001) by H2 (dihydrogen) and the occurrence of GSB (giant step bunching) in Ar-H2 mix gases system at low pressure. The steps width after our classical process, under Ar environment, is typically lower than 2 µm. The first results showed that the surface of the SiC samples annealed under Ar-H2 environment has a homogeneous distribution of well-aligned steps with terraces width up to 20 µm. A clear effect of H2 on steps formation is evidenced. However, the samples obtained had no graphene on top. Adding an isotherm of 900 s at 1500°C, and stopping the H2 right after, led to the growth of graphene on large steps, with an average steps width of 11 µm. Both optical microscopy and atomic force microscope were used to characterize the surface morphology and steps width. The presence of graphene was investigated using Raman maps. The number of graphene layers were estimated using the ratio of the integrated intensity of the G band of graphene on the integrated intensity of the G band of a highly oriented pyrolytic graphite (HOPG) used as reference (A_G-Gr/A_G-HOPG). Raman maps demonstrate that our samples are mostly covered with 1LG (up to 70% of the surface).

Metal/semiconductor interfaces are essential to build devices for fabrication of electrical contacts. However, Schottky-Mott physics, a fundamental principle governing metal/semiconductor interfaces, is not applicable to metal/semiconductor interface in devices due to Fermi level pinning, affected by processings. Two-dimensional (2D) materials as contacts for semiconductor devices have attracted great attention due to minimizing Fermi level pinning. Schottky–Mott physics has widely employed to design 2D material-based electrodes and to elucidate the contact behavior. Nevertheless, the interface between 2D and three-dimensional (3D) semiconducting materials cannot be fully understood by Schottky-Mott physics. Charge transfer and interlayer coupling have been reported in various 2D/3D heterostructure. The phenomena also affect metal/semiconductor interface. In the presentation, we performed an integrated study of theoretical prediction and experimental observation of the electrical properties of the heterostructure composed of metallic NbSe₂ and semiconducting ZnO. The density function theory (DFT) calculations predicted that charge transfer between ZnO and NbSe₂ lowers barrier height at the heterojunction and conductive surface states of ZnO provides conduction channel in the ZnO/NbSe₂ heterostructures. Crystalline ZnO/NbSe₂ heterostructures were prepared by hydrothermal method. The surface characteristics of NbSe2 were investigated by X-ray photoelectron spectroscopy. Electrical characterizations of the ZnO/NbSe₂ heterostructures showed Ohmic-like behavior as predicted by the DFTcalculations, opposed to the prediction based on Schottky–Mott model.

Low dimensional heterostructures present unique opportunities to garner tailored properties with applicability in electronics, optoelectronics, biomedicine, photovoltaics, and electrocatalysis. Here, we demonstrate that one-dimensional (1D) graphene nanoribbons (GNRs) are versatile substrates with highly adaptable work function, carrier density, and mobility that can be engineered to create unique interdimensional heterostructures with not only superior electrocatalytic activity but significantly enhanced selectivity. Tuning the surface chemistry and morphology of GNRs, which leads to the idealization of its electronic properties, results in highly enhanced inherent electrocatalytic activity for GNRs towards hydrogen evolution reaction (HER), the best reported for GNRs. Subsequently, we demonstrate that by selecting and combining with a suitable zero-dimensional (0D) counterpart, the electrocatalytic activity of GNRs can be intimately and selectively enhanced and tailored towards targeted electrocatalytic reaction. For example, while the formation of a GNR-based 0D/1D heterostructure with molybdenum disulfide quantum dots (MoS₂ QDs) led to an HER activity comparable to benchmark Pt catalyst, the association with fullerenes (C₆₀) lead to considerable augmented activity towards Nitrogen reduction reaction (NRR). The HER and NRR activity can be further engineered by controlling the ratio between the GNR and the 0D component in the heterostructure. Through a comprehensive and complementary theoretical and experimental exploration, we found that the highly tailored electronic properties and the formation of specialized interdimensional catalytic sites are responsible for the unique catalytic performances demonstrated by the heterostructure.

There has been significant interest in germanium-tin (GeSn) as a promising silicon-compatible group IV direct bandgap material for electronics and photonics. However, the maximum tin solubility in bulk GeSn solid solutions is only ~1 at% tin compared to the >10 at% tin compositions required for a direct bandgap material.¹ The metastability of high tin content GeSn limits the thermal processing window for film deposition and post-growth annealing of epitaxial layers.^{2, 3} Previous reports on annealing results of strained germanium-tin epitaxial layers have shown undesirable surface segregation of tin. We report X-ray photoelectron spectroscopy results from in-situ and ex-situ annealing of core/shell Ge/Ge_1-xSn_x nanowire assemblies. Previously, we have shown the elastic compliance of small diameter Ge core nanowires in the core/shell structure produce largely strain-free Ge_1-xSn_x epilayers.⁴ Utilizing the variable strain control of this geometry, we investigate the annealing characteristics of the GeSn surface and native oxide for tin contents in the range of 2 at% - 12 at% in both air-exposed and oxide-free nanowires. For oxidized samples, we show the presence of a tin-rich oxide that exhibits a composition dependent temperature for thermal decomposition via in-situ annealing. By varying the shell thickness, we control the compressive strain in the shell enabling examination of phase separation behavior of the oxide-free, as-grown GeSn surface via ex-situ rapid thermal annealing and air-free transfer. Additionally, we have examined the annealing effects on bulk tin distribution and local surface structure via photoluminescence, X-ray diffraction, and high resolution transmission electron microscopy.

¹M. Seifner, A. Dijkstra, J. Bernardi, A. Steiger-Thirsfield, M. Sistani, A. Lugstein, J. Haverkort, S. Barth, “Epitaxial Ge_0.81Sn_0.19 Nanowires for Nanoscale Mid-Infrared Emitters,” ACS Nano 13, 7 (2019), 8047-8054.
²S. Wu, L. Zhang, B. Son, Q. Chen, H. Zhou, C. Tan, “Insights into the Origins of Guided Microtrenches and Microholes/rings from Sn Segregation in Germanium-Tin Epilayers,” J. Phys. Chem. C 124 (2020), 20035-20045.
³W. Wang, L. Li, Q. Zhou, J. Pan, Z. Zhang, E. Tok, Y. Yeo, “Tin surface segregation, desorption, and island formation during post-growth annealing of strained epitaxial Ge_1-xSn_x layer on Ge(001) substrate,” Applied Surface Science 321 (2014), 240-244.
⁴A. Meng, M. Braun, Y. Wang, C. Fenrich, M. Xue, D. Diercks, B. Gorman, M. Richard, A. Marshall, W. Cai, J. Harris, P. McIntyre, “Coupling of coherent misfit strain and composition distributions in core-shell Ge/Ge_1-xSn_x nanowire light emitters,” Materials Today Nano 5 (2019), 100026.

Perovskite materials and 2D materials are 2 promising materials for future optoelectronic applications. Perovskite materials are having great optoelectronic properties such as high PL quantum yield, great absorption coefficient, easy synthesis at relatively low temperatures, tunable bandgap, high carrier mobilities to name a few. These properties make them shine in device applications like photovoltaics and LEDs. 2D materials on the other hand offer an arsenal of materials with interesting optoelectronic properties. Their bandgap range from metallic to insulator. With emerging properties in the monolayer regime, which sets them apart from their bulk counterparts, they offer a rich field of study.
Here in this work, we focus on synthesizing these materials and making heterostructures out of these two classes of materials. We then explore their optoelectronic properties.

A high-mobility electron gas was discovered at the interface between two oxide insulators LaAlO3 (LAO) and SrTiO3 (STO) at the LAO/STO heterostructure by Ohtomo and Hwang in 2004 [1].
We are investigating the Ba0.8Sr0.2TiO3/LaMnO3 (BSTO/LMO) heterostructures, where all layers are insulators and are neutral. The BSTO layers have a ferroelectric polarization due to Ti atoms displacements out of octahedron central position. Below 230 K the heterostructure BSTO/LMO demonstrate high conducting qwasi-metallic behavior [2]. In Ba0.8Sr0.2TiO3/La2CuO4 heterostructures the superconducting behavior was found [3]. Now we present new experimental results of the resistivity and photoresistivity measurements and of the effect of infrared, green and ultraviolet laser light illumination of the BSTO/LMO heterostructure.
The temperature dependence of the electrical resistance has been measured by the four-probe technique for BSTO/LMO heterostructures formed by LaMnO3 single crystals and epitaxial 350 nm thick films of ferroelectric Ba0.8Sr0.2TiO3 deposited on the surface of single crystal by reactive sputtering of stoichiometric targets using RF-sputtering method at 650 C [2]. Contact pads were created using silver paste connecting fine gauge golden wires to the surface of the sample. The sample was placed on a copper plate inside a Janis St-100 cryostat purged with dry nitrogen gas. The temperature was stabilized using a Lakeshore 335 temperature controller. Light irradiation was carried out with an amplified Yb-doped fiber laser system producing pulses at 3 kHz with ~200-fs duration and mean power density of 80 mW (cm-2) at fundamental wavelength of 1028 nm, as well as doubled and quadrupled frequencies at 514 nm and 257 nm. The unfocused Gaussian laser beam was 4 mm in diameter.
We investigate the effect of illumination on Ba0.8Sr0.2TiO3/LaMnO3 heterostructure electrical resistance below 230 K, where the heterostructure demonstrate high conducting behavior. We conducted time-resolved photoresponse experiments by periodically turning the illuminating laser on and off and recording the response signal. In our case the response signal was the resistance change. The effect of green and ultraviolet laser light on Ba0.8Sr0.2TiO3/LaMnO3 heterostructure electrical resistance was studied. In 80-210 K range illumination induces increasing of the resistance at ~+15% of the steady-state value, the time constant associated with the transient component is ~4-12 s. The negative photoresistance effect is found under green, infrared and ultraviolet illumination.
Thus at low temperatures (T<~210 K) as the illumination starts, the electrical resistance begins to grow with a time constant of ~12 s and then it reaches the steady state value while the sample is continuously illuminated. This behavior is opposite to usual photoconductivity in ordinary semiconductors where due to photo-carriers the conductivity increases. Thus, the negative photoresistance effect is detected for green, infrared and ultraviolet illumination. The observed effect cannot be explained by the direct heating of the sample by laser pulses, because the laser pulse repetition rate is low and therefore the cumulative thermal effects should be negligible. And also, the current, which are related to our resistivity for these temperature, flow on interface area, but not on the surface. This example shows that quasi-two-dimensional high conductance at ferroelectric/dielectric interfaces can be controlled by light.
The reported study was funded by Russian Science Foundation according to the research project No. 21-12-00179.
REFERENCES
1. A. Ohtomo and H.Y. Hwang, Nature 427, 423 (2004).
2. D.P. Pavlov et al., JETP Letters 106, 460 (2017).
3. D.P. Pavlov et al., PRL 112, 237001 (2019).

Incorporating a heterojunction architecture into photodetectors can significantly improve the photodetection characteristics. To construct a heterojunction with the nanostructured compound semiconductor materials (CSMs), a two-dimensional (2D) platform material for stably transferring photogenerated signals into circuitry is essential. In this work, p-type manganese oxide quantum dots (MnO QDs) with zero-dimension (0D) are synthesized through femtosecond laser ablation in liquid (FLAL). An exfoliated 2D molybdenum disulfide (2D-MoS₂) is used for the counterpart platform of the heterojunction. The ablated MnO QDs with a mean diameter of 4.9 nm were uniformly spray-coated onto the 2D-MoS₂. The p-n junction device based on p-MnO QDs functionalized 2D n-MoS₂ shows a reduced dark current and an increased photocurrent compared to that of pristine 2D-MoS₂ devices due to the p-n heterojunction effect. This report will greatly contribute to developing low-cost and high-performance optoelectronic devices based on heterojunctions between other TMDCs and nanostructured CSMs.

Chemical and biological sensing using 2-D nanomaterials has been an area of intense investigation including inorganic materials that exhibit high sensitivity, however it has been limited as a field due to the lack of selectivity among analytes. While 2D inorganic materials, such as transition metal dichalcogenides (TMD), can readily decipher between donor/acceptor groups, strategies to incorporate selectivity in these devices is often challenging. Organic 2D materials known as covalent organic frameworks (COF) are large porous and repeated organic frameworks which have been shown to act as selectivity agents due to functionalization and porosity. This research focuses on the association of COFs with 2-D nanomaterials using top-down microwave-assisted activation of TMDs and deposition of COF nanoparticles. Several known TMD and COF species have been examined to understand the fundamental surface interactions between 2-D nanomaterials and few layer COFs, while Kinetics and mechanistic studies were used to determine the manner of association of the inorganic and organic substrates. Photoluminescence studies demonstrate a microwave power dependent electronic coupling of the inorganic and organic materials for a fully tunable interaction.

Emerging twisted heterostructures based on 2D layered materials have opened a unique platform to explore exotic correlation physics and engineer Coulomb interactions in the atomically thin limit. The tunable Moiré potentials, arising from interlayer couplings, result in the emergence of various novel orderings such as Mott insulating phases, unconventional superconductivity, and Moiré excitonic phases. In particular, transition metal dichalcogenide heterostructures allow for optically-excited electrons and holes to reside in different monolayers and for trapping in the tens-of-meV Moiré potential, which is quite favorable for developing valleytronics, exciton condensation, and programmable quantum emitter arrays. Further progress along this direction requires a precise understanding of the formation of these Moiré excitons, and how such formation can be engineered by Moiré potentials and electron correlations.
Here we use ultrafast terahertz emission spectroscopy to probe interlayer charge transfer and other quasiparticle dynamics in several types of Moiré superlattices based on the transition metal dichalcogenides. With ultrafast visible and near-infrared laser pumping of these distinct Moiré superlattices, we observed nontrivial variations in the THz emission dynamics and spectroscopy as a function of twist angle. Experiments indicate such changes are closely associated with varying Moiré patterns and momentum mismatch conditions. Our findings advance our microscopic understanding of the formation, dissociation and manipulation of excitonic correlations in twisted heterostructures.

A route for synthesis of epitaxial pentasodium trioxoundecafluoro-trititanate (Na₅(Ti₃O₃F₁₁)) with plate-like morphology on fluorine-doped tin oxide coated glass substrates (FTO- glass) through the hydrothermal bath method is described and discussed, followed by the presentation of a systematic study of the physical properties of the samples. The samples are textured with a small amount of micron sized crystals existing among predominantly plate like structures of Na₅(Ti₃O₃F₁₁). These structures are attached to the F:SnO₂ (FTO) layer due to their nucleation and growth on the substrate surface during the hydrothermal process. The preferred orientation of the crystal planes of Na₅(Ti₃O₃F₁₁) plates are with the (331), (222) and (440) parallel to the substrate. Additionally, an epitaxial relationship between the fluorine doped SnO₂ layer and plate-like (Na₅(Ti₃O₃F₁₁)) exists: an alignment of planes along (111). Last of all, a set of unidentified vibrational Raman modes recorded from Na₅(Ti₃O₃F₁₁) plates grown on fluorine-doped tin oxide is presented. It is found that in the temperature ranging from 27 to -193 ^oC, Na₅(Ti₃O₃F₁₁) vibrational Raman modes peak at: 118.1 cm^-1, 150.9 cm^-1, 191.0 cm^-1, 237.4 cm^-1, 439.5 cm^-1, 527.1 cm^-1, 642.8 cm^-1, 687.9 cm^-1, 742.6 cm^-1 and 943.5 cm^-1. Raman shifts at 527.1 cm^-1, 687.9 cm^-1, 439.5 cm^-1, and 943.5 cm^-1 are assigned to A_g, B_1g, B_2g, and B_3g modes, respectively.

Magnetic nanowires (MNWs) are increasingly important for applications in memory, biolabels, and nanowarming of cryopreserved organs. Object Oriented Micromagnetic Framework (OOMMF) was used to visualize moments inside MNWs. The magnetic properties of MNWs, such as coercivity, remanence, and reversal mechanism were investigated as a function of nanowire shape. Experimental measurements of magnetic nanowires (MNW) often reveal features that are difficult to explain due to statistical shearing of features and interaction fields in large arrays. Measurements of individual nanowires can be used to remove interaction field effects, and other measurements such as first-order reversal curves (FORC) and the projection method often attempt to statistically separate bulk effects. For nickel MNWs with fixed length of 3 μm, the coercivity and the remanence are negatively correlated to the diameter of the wire. This is in consistent with the theory that expresses the coercivity field of the infinite long cylinder as: h_ci=1.08D₀²/D², D₀=2A^0.5/M_s, where h_ci is the reduced coercivity field of the cylinder with high aspect ratio, D₀ is the fundamental unit length defined by the exchange constant A and saturation magnetization M_s of the material (Ni in this case) used to make MNWs.
When the diameter is increased to 200 nm, in addition to a continued reduction in coercivity and remanence, a ‘wasp waist’ appeared in the hysteresis loop. The reversal can be divided into three different parts showing three types of magnetic moment behaviors in the MNW. In the first part, since there is a large area for curling in 200nm-diameter nanowires, the magnetic moments at the tips of the MNW fall from pointing along z-axis to aligning in the xy-plane and in the form of vortex. Next, when the external field is further reduced, the outer shell of moments turns from in-plane to the negative z-axis. Finally, the axial core of the MNW switches to the negative z-axis and the wire is fully saturated in the negative z-direction. Interestingly, at H_c (M = 0), the two vortex walls converged in the center of the MNW, and a three-dimensional vortex(hedgehog) was observed.
Next, with all other parameters unchanged, flat-tipped cylinders were replaced with ellipsoids so that the tips of wire had infinitesimally small areas where vortex was no longer supported. With this shape, the coercivity of the nanowires significantly increased to Hc = 180, 160, 50, 11 mT from 140, 90, 30, 5 mT for the MNWs with 30, 50, 100, 200nm diameters, respectively. This increase in coercivity occurs because the field to nucleate a vortex at the tips is lower than to nucleate vortex in the center of the wire which is the case for ellipsoidal MNWs. For these ellipses, increasingly negative applied fields cause the vortex to expand to include the coherent curling of the whole nanowire.
In conclusion, the coercivity of the nanowire depends on the geometry of the wire in two ways. First, with the length of the wire fixed, the coercivity is reduced by increasing the diameter of the wire. Second, when the tip of the wire is engineered so that the nanowires are ellipsoidal rather than cylindrical, the coercivity is increased and the moment switching begins at the center of the wire instead of the end.

(Ba_0.85Ca_0.15) (Zr_0.1Ti_0.9) O₃ (referred as BCZT), a solid solution of BaTiO₃, is a novel lead-free ferroelectric relaxor that possesses a high dielectric constant, large piezoelectric coefficient as well strong electro-optic coupling. High quality BCZT thin films have been successfully synthesized by various techniques, including the epitaxial growth using pulsed laser deposition (PLD) recently by our group. However, the requirement of lattice matching to substrates and the extremely high material growth temperature (up to 800°C) are extensively limiting the practical applications in Si-based electronics integrations, as well as the emerging flexible or stretchable devices that usually use polymer as supporting substrates. Here, we report the synthesis and transfer of freestanding defect-free ferroelectric BCZT membranes via epitaxial lift off from growth substrates using LSMO as sacrificial layer. Ferroelectric and piezoelectric properties of transferred film are comparable to the as-grown epitaxial BCZT film. Furthermore, 10×10 SRO/BCZT/SRO capacitor array were fabricated using wet chemical etching method and further transferred onto alien substrate as a demonstration of potential device application. Our work paves the way of integrating single-crystalline relaxor ferroelectric oxides for on-chip electronic or energy storage applications.

Crosslinked polyamides are a diverse and important group of polymers with many applications in the membrane and desalination fields. These polymers are usually synthesized by interfacial polymerization and supported over a porous polymer support that provide mechanical stability. In the last years, considerable effort has been put to make thinner and stronger polyamide films that can have higher water permeability yet mechanical robustness. The combination of thin films with one-dimensional (1D) reinforcing structures is an effective path to combine the functionality of the film with the structural stability provided by the 1D material. Indeed, this type of structures are so efficient that have resulted from evolution in many living creatures, and can be seen, for example, in insect wings and plant leaves. However, taking this design to the nanoscale poses great challenges. Ultrathin polymer films, sometimes referred as quasi two-dimensional (2D) materials, often show processing problems such as dewetting, phase separation, and creeping, among others. Nevertheless, the study of these materials has recently gained momentum. Here, we used tempo-oxidized cellulose nanofibers (CNFs) and double walled carbon nanotubes (DWCNTs) to synthesize thin and ultrathin (<30 nm) crosslinked -piperazine and phenylendiamine based polyamides films by interfacial polymerization. Tempo-oxidized CNFs are high-modulus water-dispersible colloids. They are biodegradable and its synthesis is environmentally friendly. On the other hand, DWCNTs are a superhigh modulus material that might have interesting transport properties due to their hydrophobic hollow core. We carried out liquid-liquid interfacial polymerizations to prepare sheet like quasi 2D polymer sheets reinforced with CNF, DWCNT and their combination. The mechanism of the interfacial polymerization in presence of the nanomaterials was also studied by using different methodologies to add the monomers. We use several techniques such as quartz-crystal microbalance, transmission and scanning electron microscopy to study how hydrophobic and hydrophilic 1D materials can be efficiently incorporated into interfacially synthesized polymer films. The location of the 1D materials across the films was also studied by X-ray photoelectron spectroscopy. The chemical interactions responsible for wetting, interfacial adhesion, and phase separation between the monomers and the 1D materials were studied at the atomic level through classical molecular dynamics. Understanding the incorporation of 1D materials and quasi 2D polymer films and the behavior of the resulting nanocomposite films is important to design new ultrathin nanocomposite polymers that might have applications for the next generation of separation membranes.

TiO₂/SrTiO₃ heterostructured nanorod arrays have been attracted much attention in photovoltaic and/or photoelectrochemical (PEC) applications such as dye-sensitized solar cells (DSSCs) and PEC cells due to its unique feature of oriented 1D structure and heterostructure. In this context, we designed anatase TiO₂/SrTiO₃ heterostructured nanorod arrays grown on transparent conductive fluorine-doped tin oxide (FTO) substrate and the comparison of the characteristic properties with those of typical rutile TiO₂/SrTiO₃ heterostructured nanorod arrays. Furthermore, novel niobium doped TiO₂/SrTiO₃ heterostructured nanorod arrays were also synthesized to investigate the compensation for the low conductivity of TiO₂ and SrTiO₃ themselves.
TiO₂/SrTiO₃ heterostructured nanorod arrays were synthesized by two-step hydrothermal reaction. Oriented 1D structure is formed by the growth of anatase or rutile TiO₂ nanorod arrays on FTO substrate directly during the first hydrothermal reaction, and TiO₂/SrTiO₃ heterostructure is constructed by a dissolution-precipitation process on TiO₂ nanorod arrays through the second hydrothermal reaction by changing the reaction conditions such as reaction temperature, reaction time, the initial concentration of the starting materials, and basicity of the reaction mixture. As the typical method, the obtained crystal structure of TiO₂ nanorod arrays is mainly rutile phase, which is the most stable phase among the natural phases of TiO₂. Formation of rutile phase of TiO₂ sometimes restricts the formation of SrTiO₃ due to the higher chemical stability of the rutile TiO₂. The morphologies and chemical components of TiO₂/SrTiO₃ heterostructured nanorod arrays were characterized by the measurements of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible (UV-vis) spectroscopy. The amount of strontium captured in anatase TiO₂ lattice was larger than that of strontium captured in rutile TiO₂ lattice. The extent of the increment of the average diameter of the nanorod before and after the second hydrothermal reaction was also different between the two crystal phases of TiO₂. It is suggested that better dissolution of anatase TiO₂ resulted in the larger amount of participation of SrTiO₃ and increase of the nanorod diameter effectively.
Niobium doping to TiO₂/SrTiO₃ heterostructured nanorod arrays was conducted by the Sr²⁺ insertion into the Nb-doped TiO₂ nanorod arrays. The existence of niobium was confirmed by XPS and scanning transmission electron microscopy (STEM) measurements. We will report the further regulation and optimization for formation of Nb-doped TiO₂/SrTiO₃ heterostructured nanorod arrays by changing the preparation conditions and crystal structures of TiO₂ in terms of the improvement of the conductivity.

This work reports a theoretical study of the nonlinear optical and electronic properties in oblate core/shell quantum dots SiGe in the presence of a donor impurity under electric and magnetic field. Our computations were carried out within the effective mass approximations. For the numerical solution of the resulting 3D partial differential equation (PDE) via finite element method. A detailed study of the donor binding energies as a function of the external electric and magnetic fields, the dipole matrix elements for impurity related inter-level optical transitions as a function of the electric and magnetic fields and impurity position, the absorption coefficient, second (SHG) and third harmonic generation (THG). Our results show that the electric and magnetic fields have significant influences on the ground binding energies and the absorption coefficient, second (SHG) and third harmonic generation (THG). The effect caused by electric and magnetic fields and on-center of shell donor impurities, ostensibly enrich the nonlinear and electronic properties of the system.

The state-of-the-art epitaxial growth technique of group III nitride semiconductors can offer a wide range of the applications, such as light emitting diodes, laser diodes, and high-frequency power devices. Devices usually fabricated on the polar c-plane GaN. Therefore, the optical devices fabricated on the plane show inferior performances from the Stark effect. On the other hand, a-plane GaN, which is non-polar plane, will have advantage on the production of superior optical devices. A-plane GaN is usually grown on an r-plane sapphire, although the density of threading-dislocations is still very high. Remoteepitaxy [1] is newly proposed growth method, where growth is performed on graphene-coated substrate. The technique is expected to mitigate the lattice mismatch and the thermal stress produced from the difference between the growth layer and the substrate. A high-quality a-plane GaN crystal is expected to be grown by the method. The grown layer can also be peeled off from the substrate and suitable to make flexible devices. In addition, the reuse of the substrate is possible, which may lead to the sustainable development goals. In this study, we attempt a hetero remoteepitaxy to grow a-plane GaN on graphene/r-plane sapphire. The effect of growth temperature on the crystal alignment of GaN is especially focused on to study.
Single-layer graphene was synthesized on an r-plane sapphire substrate using low-pressure CVD [2]. GaN layer was grown using a radio frequency molecular beam epitaxy directly on the graphene-covered sapphire substrate using hetero remoteepitaxy. The nitrogen flow rate, RF plasma power, Ga equivalent pressure, and growth time were fixed at 2.0 sccm, 450 W, 4.0 × 10^-7 torr, and 10 min while the substrate temperature was changed between 630 and 700 °C. The shape of GaN growth islands was investigated using atomic force microscopy.
C-plane GaN islands with hexagonal shape were observed on the sample grown at 630 °C. On the other hand, c-plane GaN islands decreased with the growth temperature and disappeared on the sample grown at 675 °C, and a-plane GaN islands generated instead of them. The crystal orientation of the a-plane GaN islands was found to be aligned to a certain direction from their rectangular shapes. Surprisingly, two-dimensional nuclei of GaN also started to generate on the sample grown at 700 ° C. A thin GaN layer was also successfully grown whose thickness was as thin as 20nm in the condition. Namely, the hetero remoteepitaxy was succeeded above 675 °C because the crystal orientation of the rectangular a-plane islands aliened to a certain direction. On the other hand, the hexagonal c-plane islands didn’t align at 630 ^oC. One of the possible reasons of the success is the change of the gap between graphene and the substrate. The gap may be shorten by the removal of some atoms, such as oxygen or hydrogen, from the surface of the sapphire during the thermal treatment. In conclusion, the growth temperature was found as a very important parameter for a-plane GaN remoteepitaxy, and even two-dimensional nucleation was successfully obtained at 700 ^oC.
[1] Y. Kim et al., Nature 544, 340 (2017). [2] Y. Ueda et al., Appl. Phys. Lett. 115, 013103 (2019).
Acknowledgements: This work was supported in part by JSPS KAKENHI Grant Numbers 25000011, 15H03558, 26105002, and 2660089.

The structural and electrical properties of carbon nanotubes (CNTs) and Copper (Cu) based nanostructure is presented keeping in mind their applications today and in the near future. CNTs are emerging as the most promising conducting wires in interconnection technology due to their higher electrical conductivity and extraordinary current-carrying capacity. Increased portability, versatility and ubiquity of electronics devices are a result of their progressive miniaturization, requiring current flow through narrow channels. Nowadays, devices operate close to the maximum current-carrying-capacity of conductors (such as copper and gold), leading to decreased lifetime and performance, creating demand for new conductors with higher ampacity. Therefore, researchers exploring electrical properties of CNTs/Cu composites by changing the concentrations of Cu content and vice versa. CNTs/Cu composites shows high conductivity and ampacity, making it uniquely suited for applications in microscale electronics and inverters.

Two-dimensional layered materials that harvest or transduce light energy are of paramount interest. In this context, two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS₂) are attractive for harnessing optoelectronics. In this work, we report the integration of MoS₂ with a strong electron accepting graphene yielding novel MoS₂ (1-2L)/graphene (1-2L) heterostructure ensemble by facile and efficient dry transfer method. We also found additional hetero-interfaces due to folded flakes such as MoS₂/MoS₂/graphene and MoS₂/MoS₂/Au hetero-interfaces. We characterized the heterointerfaces using correlated micro-Raman spectroscopy, photoluminescence (PL) spectroscopy, atomic force microscopy, Kelvin probe microscopy and conducting AFM, tip-enhanced Raman spectroscopy (TERS), and tip-enhanced photoluminescence (TEPL) techniques to gain insights into the structural quality, hetero-interfaces, and excitonic effects to elucidate the role of a number of graphene layers which are crucial in modulating the light emission. We studied the charge transfer in these vertical heterostructures unraveling the interlayer electronic coupling. We demonstrate strong evidence that the excitonic properties of MoS₂ are most effective for two layers of graphene layers underneath leading to a significant enhancement of the PL response as compared to MoS₂ supported on one-layer graphene and directly on the gold-coated substrate. We observed a marginal increase in work function with increasing MoS₂ layer as anticipated and for heterointerfaces which are indicative of electron injection. We measured C-AFM without light (dark) and with light in view of photoconducting properties of heterointerfaces and found a rectifying behavior with an apparent increase in current by almost twice for MoS₂/graphene heterostructures in contrast to MoS₂/Au which shows ohmic behavior. Our findings signify the importance of substrate engineering when constructing 2D multilayered architectures for harvesting or storing light energy. This work is financially supported in parts by NSF and NSF-MRI Grants.

Van der Waals (vdW) heterostructures composed of two-dimensional (2D) semiconducting material stacks have garnered significant attention owing to their unique optoelectronic properties compared with their single constituent counterparts. Especially, heterostructure devices composed of 2D semiconductors of which the interfacial exciton transport is controllable hold great potential in photonic and optoelectronic applications. Here, we report vdW heterostructures made of PbI₂ and MoSe₂ and demonstrate that the local exciton distribution can be controlled by modulating the vdW interface between them. PbI₂ vdW layers are prepared in one-dimensional (1D) morphologies with discrete stacking orientations and transferred onto the 2D MoSe₂ layers grown via the chemical vapor deposition (CVD) method. Specifically, the vapor-liquid-solid (VLS) method was employed to grow 1D PbI₂ vdW nanostructures in two different layer stacking orientations (i.e., [001] and [010]) and layer stackings (perpendicular vs. parallel to the wire axis). Upon the transfer onto MoSe₂, we confirm that the exciton transfer between the two materials is strongly dependent on the interlayer configurations (i.e., layer-to-layer vs. edge-to-layer). Strong exciton binding energy of PbI₂ enables the transport of excitons along with the PbI₂ domain via waveguide effect and localized radiative recombination according to the interface with the MoSe₂ domain. Our results provide important insights into the engineering of vdW heterostructure interfaces and develop a new design rule of ultrathin optoelectronic devices.

The last decade has seen an exponential growth in the science and technology of two-dimensional materials. Beyond graphene, there is a huge variety of layered materials that range in properties from insulating to superconducting that can be grown over large scales for a variety of electronic devices and quantum technologies, such as topological quantum computing, quantum sensing, and neuromorphic computing. Over the last few years, we have focused on understanding what happens to traditional bulk metals (Ga, In, Ag, Au, etc.) when they are reduced to to only a few atoms thick. In this talk, I will discuss recent breakthroughs in the realization of unique 2D forms of these traditional 3D metals. I will discuss a novel synthesis method, dubbed confinement heteroepitaxy (CHet), that utilizes graphene to enable the creation of atomically thin metals, enabling a new platform for creating artificial quantum lattices consisting of 2D and 3D materials, with atomically sharp interfaces and designed properties. By shrinking these traditional metals to atomically thin structures, we find that their properties are completely different than their bulk counterparts, lending themselves to unique quantum and optical applications not possible before.

References
Nature Materials 15, 1166–1171
Nature Materials 19 (6), 637-643
Advanced Funct. Materials, 2005977
Nano Letters 20 (11), 8312–8318
Nanoscale 11(33), 15440-15447

Scientists around the globe are investigating how to integrate distinct 2D materials into van der Waals (vdW) heterostructures. Given that any passivated, dangling bond-free surface will interact with another via vdW forces, the vdW heterostructure concept can be extended to include the integration of 2D materials with non-2D materials. Such combinations of materials would yield vdW heterostructures showing a variety of structural and electronic properties that opening up new avenues for innovative device designs. This presentation will focus on three types of mixed-dimensional vdW heterostuctures. We successfully employed graphene/QDs (2D/0D) and graphene/MoS₂ (2D/2D) in their low dimensional heterostructures to form photodetector devices. Those devices showed novel functional behaviors, which are discussed based on electrical/optical characterizations and possible physical mechanisms. The third mixed-dimensional, 2D/3D, vdW heterostructure will be discussed on the basis on the growth study of (In)GaN on graphene for the preparation of relaxed templates for micro-display applications.

Graphene with “superclean” surface is prepared via selective etching in carbon dioxide (CO2) gas. Gentle etchant carbon dioxide selectively eliminate intrinsic contamination such as amorphous carbon during a post annealing process[1]. Is the etching process required as a post annealing?

We used CO2 as an atmospheric gas during carbon film deposition. Carbon films were prepared in carbon dioxide atmosphere by pulsed laser deposition (PLD) on insulating substrates; strontium titanate (SrTiO), sapphire(Al2O3) and silicon (Si) substrates. Target was a highly oriented pyrolytic graphite (HOPG), and slower Q-siwtched YAG was used for PLD system[2].


Graphene grew only on the STO substrate, and showed clean surface image observed by atomic force microscopy (AFM)[2]. CO2 is a colorless and stable gas, but CO2 can be a gentle oxidant with reaction with carbon as,

CO2 + C -> 2CO.

As the same as “superclean” surface prepared via selective etching in CO2 atmosphere, assumed carbon cluster etched by CO2 and only graphitic materials reach the substrates. Stability of those cluster on the substrate surface was evaluated by absorption energy between the cluster and substrate surface.

In order to estimate an absorption energy of graphitic materials on insulating substrates, a carbon atom, six membered ring (6-ring), and 6 six membered rings (nano graphene) were placed on SrTiO, silicon and sapphire substrates, and the absorption energy was estimated by using the density functional theory (DFT) with a semi-core pseudopotential. The generalized gradient approximation (GGA) method was used to obtain the electron density. The energy difference for self-consistent field was set at 1.0x10-6 Ha. Crystal structure of each substrates was optimized with periodic condition, and supercell consisting of slab and vacuum layer was used to optimize the top layer of substrate. After optimal surface was prepared on each substrates, supercell with graphitic cluster and slab was optimized, and total energy was estimated on supercell combined cluster and substrate (Etot_combined).

Absorption energy was evaluated to be

Etot_sub + Etot_cluster - Etot_combined,

where Etot_sub and Etot_cluster) were total energy of subs and graphitic cluster, respectively.

The absorption energy of graphene on SrTiO(001) and Si(001) were estimated to be 570 and 550 kJ/mol, respectively. Although graphene grew on SrTiO substrate while DLC related materials on Si(001), there was not much difference in absorption energy between SrTiO and Si substrates. However form of six-ring on each substrates was remarkably different after optimal combined structure. After optimizing combined structure of six ring placed on Si(001), the six ring did not lie on silicon substrate but it stood up vertically on the substrate while a six ring flatly lied on SrTiO substrate. Actually on the silicon substrate, AFM image revealed the growth of nano balls on the surface while graphene grew with “layer by layer growth” on SrTiO substrate. Cluster generated from target HOPG can be etched and graphitic materials reached to the substrate. However film growth was affected by stability of the cluster placed on substrates because graphene grow from graphitic materials such as nano-graphene and six membered ring. Evaluation of variety of combination of carbon materials and substrates is in progress.

[1] J. Zhang et al., Angew. Chem. Int. Ed. 58, 14446 (2019).
[2] S. Kaneko et al., Jpn. J. Appl. Phys. 40, 4870 (2001).
[3] S. Kaneko et al., ACS Omega 2, 1523 (2017).

Vertically stacked 2D materials heterostructures show potential applications in next-generation nano-electronic devices due to atomic-scale thickness, ultrafast charge transfer, scaling flexibility and precise tuning of atomic band alignment. Tellurene, a recent addition to 2D materials family, has shown promising aspects in nano-devices due to its favorable properties like good ambient stability, high carrier mobility, excellent light absorption, and exceptional thermoelectric performance.^1,2 Atomistic understanding of the electronic properties of tellurene-metal interface is crucial to promote optimal device performance. Pristine monolayer (ML) β tellurene is semiconducting in nature and possesses an indirect bandgap of about 1.27 eV.³ When monolayer tellurene is interfaced with metals, the bandgap of tellurene completely disappears and it becomes metallic from semiconducting.⁴ This phenomena occurs for all metals that are commonly used as electrode materials, restricting the use of tellurene in vertical Schottky barrier devices. In a recent work, we have shown that inserting single layer of graphene between tellurene and metal can preserve the semiconducting nature of tellurene and hence useful to form Schottky barriers.⁵

In this work, utilizing density functional theory (DFT) we have evaluated the effect of inserting monolayer hBN between tellurene and metal. Using four metals (Au, Pd, Ru, Ti) we have shown that inserted hBN layer can also screen the metallization of tellurene and create vertical Schottky barrier. DFT calculations have been carried out using Vienna ab-initio simulation package (VASP) with Projector Augmented Wave (PAW) method and GGA-PBE exchange correlation functional. To create such heterostructures, first the tellurene-hBN interface is formed by aligning the supercells of the two and applying mean strain of 1.1% in both the surfaces. The tellurene-hBN structure is then interfaced with six layers of <111> cleaved surfaces of Au, Pd and <0001> cleaved surfaces of Ru, Ti. The applied strain on Au, Pd, Ru and Ti are 0.34, 3.45, 3.53 and 1.18% respectively. The surface area of the relaxed tellurene-hBN-metal heterostructures 22.6×8.6 Ų, 8.5×23 Ų, 25.7×4.5 Ų and 8.67×22.96 Ų, respectively. From the projected band structure of tellurene, we found that the band structure of tellurene remains preserved in all the heterostructures, only the energy bands are shifted with respect to Fermi level creating vertical Schottky barriers between tellurene and hBN-metal interface. From Bader charge analysis we found that in Au and Pd-based heterostructures, the charge in tellurene layer is less than that of pristine tellurene, implying depletion of charge. As a result, the valence bands of tellurene shift towards the Femi level. In case of Ru and Ti, charge accumulation takes place in tellurene layer resulting in shifting of conduction bands towards the Fermi level. However, for Pd and Ti, the shifted bands touch the Fermi level, generating Ohmic contact. For Au, p-type Schottky barrier is formed with a height of about 0.2 eV, while for Ru the barrier is n-type with a height of 0.32 eV. In graphene inserted structures, only p-type Schottky barriers are formed for all these metals, the barrier heights change depending on the metal work-function. However, using hBN we can tune the barrier height as well as the type of contact only by changing the electrode materials. This study provides quantum-mechanical insights into the realization of high-performance vertical Schottky devices using monolayer tellurene based heterostructures.

References:
1. Sang, D. K. et al. Nanomater. (Basel, Switzerland) 9, (2019).
2. Wang, Y. et al. Nat. Electron. 1, 228–236 (2018).
3. Wu, B. et al. Mater. Res. Express 4, 95902 (2017).
4. Yan, J. et al. J. Mater. Chem. C 6, 6153–6163 (2018).
5. Mitra, S. et al. J. Phys. Chem. C 125, 12975–12982 (2021).

Ferroelectric polymers such as poly(vinylidene-fluoride-trifluorethylene) (P(VDF-TrFE)) are promising material candidates for developing flexible wearable nanoelectronics and optoelectronics. For polarization-enabled device functionalities, large-scale fabrication of polymer thin films with high crystallinity and well-controlled polarization direction is highly desirable and remains a central challenge. The widely exploited Langmuir-Blodgett (LB), spin-coating, and electrospinning methods only yield polymorphous or polycrystalline films with compromised net polarization. In this work, we report a highly scalable approach for fabricating P(VDF-TrFE) thin films with high crystalline order and out-of-plane polarization via interface-epitaxy with 1T’-ReS₂ [1]. We deposited 1 - 9 monolayer P(VDF-TrFE) thin films on mechanically exfoliated ReS₂ flakes using the LB technique. Upon controlled thermal annealing, the as-grown LB films crystallize into close-packed (010)-oriented nanowire (P-NW) structure with radii of 10 or 25 nm. The P-NWs are well aligned along the direction perpendicular to the b-axis of the interfacial ReS₂ layer. High-resolution transmission electron microscopy studies show that the P-NWs are single crystalline in (001)-orientation and form an epitaxial van der Waals heterointerface with the underlying ReS₂. Piezo-response force microscopy studies confirm the uniform out-of-plane polarization direction of the P-NW films, and reveal nonvolatile polarization switching performance with coercive voltages as low as 0.1 V. Reversing the polarization of the P-NWs can induce a conductance switching ratio of >10⁸ in a bilayer ReS₂ channel, which is among the highest values reported on P(VDF-TrFE)-gated 2D FETs. We also carried out in-situ atomic force microscopy to image the P-NW formation process. Our study points to an easily scalable approach for achieving wafer-size production of continuous P-NW films and high performance nonvolatile ReS₂ transistors. This work was primarily supported by DOE Award No. DE-SC0016153.
[1] Li et al., Adv. Mater. DOI: 10.1002/adma.202100214 (2021).

Van der Waals heterostructures allow the integration of functionalities from different nanomaterials to achieve advanced, superimposed properties. Here, mixed MoS₂-WS₂ heterostructure grains were grown using atmospheric pressure chemical vapor deposition technique, and their optoelectronic properties were investigated. The field effect transistors fabricated from the heterostructure grains exhibited high photoresponsivity, which was influenced by the gating potential, as well as the measurement temperature. During the gating potential sweep from -5 V to +55 V, the light to dark source drain current ratio (I_light/I_dark) increased from 3 to 16 and from 2 to 7 at 60 K and 120 K, respectively. Such phenomena can be leveraged to realize novel optoelectronic and memory applications.

The ability to create and manipulate materials in two-dimensional (2D) form has repeatedly had transformative impact on science and technology. In parallel with the exfoliation and stacking of intrinsically layered crystals, the atomic-scale thin film growth of complex materials has enabled the creation of artificial 2D heterostructures with novel functionality and emergent phenomena, as seen in perovskite oxides. We present a general method to create freestanding complex oxide membranes and heterostructures with millimeter-scale lateral dimensions and nanometer-scale thickness, using an epitaxial water-soluble buffer layer [1-3]. This facilitates many new opportunities we are beginning to explore, including the topological melting transition of 2D crystalline order [4], the application of extreme tensile strain and strain gradients [5-7], and integration with other materials families and synthesis techniques [8,9].

[1] D. Lu et al., Nat. Mater. 15, 1255 (2016).
[2] Z. Y. Chen et al., Phys. Rev. Mater. (RC) 3, 060801 (2019).
[3] P. Singh et al., ACS Appl. Electron. Mater. 1, 1269 (2019).
[4] S. S. Hong et al., Sci. Adv. 3, eaao5173 (2017).
[5] S. S. Hong et al., Science 368, 71-76 (2020).
[6] R. J. Xu et al., Nat. Commun. 11, 3141 (2020).
[7] V. Harbola et al., Nano Letters DOI: 10.1021/acs.nanolett.0c04787 (2021).
[8] D. Lu et al., Nano Letters 19, 3999 (2019).
[9] D. F. Li et al., Nano Letters DOI: 10.1021/acs.nanolett.1c01194 (2021).

Long-wavelength infrared (IR) III-V based devices have long been of interest for potential applications such as chemical sensing and large format IR imaging. Within the III-V family, only the InAs_1-xSb_x bulk alloy in composition range 0.45 ≤ x ≤ 0.8, offers the required bandgap energy (E_g) from 100 to 125 meV at an operating temperature of 80K and below [1]. Over this composition range however, the InAs_1-xSb_x lattice constant varies from 6.24 to 6.39 Å, where the lack of conventional substrates has restricted progress on the growth and study of this material system. To overcome this limitation, we employ a metamorphic step-graded InAs_1-xSb_x buffer on GaSb, enabling the study of low-E_g InAs_1-xSb_x as a function of growth conditions. Using this method, we investigated the effect of substrate temperature (T_sub) and group V to group III flux ratio (beam equivalent pressure, V/III) on Sb incorporation of the lowest-E_g cap layer [2]. We also used x-ray reciprocal space mapping (RSM) to examine the effect of growth conditions on strain and dislocation dynamics. Following these growth studies, we employed the metamorphic InAs_1-xSb_x in an InAs/InAsSb superlattice designed with a cutoff wavelength of 9 µm which leads to improved absorption compared with the lattice-matched counterpart.

We first grew several InAs_1-xSb_x step-graded structures in which the Sb/(As+Sb) flux ratio was varied from 0.05 to 0.50 in 0.05 increments, under various T_sub and V/III. Nomarski imaging revealed smoother surfaces under a V/III=10, the highest ratio we attempted. At this higher V/III, we observed the cross-hatch morphology expected for metamorphic materials and found that the cross-hatch spacing changes, implying a change in dislocation dynamics, with T_sub. We also used photoluminescence (PL) to measure the Sb-content in the cap layer as well as compare intensities between samples. We found the highest Sb-incorporation to occur when T_sub=415 C and V/III=10, while the most intense samples used T_sub=415-430 C and V/III=10 [2].

We also identified the Sb composition in each layer using RSM along [110] with (004) and (115) reflections. This allowed comparison of Sb-content as a function of Sb/(As+Sb) for various T_sub and V/III. The results suggest that V/III has little effect on Sb incorporation, in direct conflict with our previous PL results [2]. To understand the discrepancy between PL and RSM, we measured (004) RSM of the same three samples with the x-ray beam incident along [1-10], revealing extremely different strain relaxation compared to the [110] case. Asymmetric strain relaxation has been observed in other III-V graded buffer systems and has been explained by different dislocation formation energies and glide velocities along each direction resulting from the core structure of the dislocation being terminated with either a group-III or a group-V element [3]. Transmission electron microscopy is ongoing to further understand the dislocation dynamics in these samples.

Ultimately, we used the lessons learned above to investigate the effect of substrate lattice-constant on strain-balanced InAs/InAsSb superlattices designed for 9 µm cutoff wavelength. Theoretically, by using a larger substrate lattice-constant the superlattice design results in larger electron-hole wavefunction overlap, ultimately increasing photon absorption. Our experimental results confirm this theory, even in the presence of increased threading dislocations.

[1] I. Vurgaftman et al. JAP 89, 5815-5875 (2001).
[2] Tomasulo et al. J. Vac. Sci. and Technol. B 36, 02D108 (2018).
[3] France et al. J. Appl. Phys. 107, 103530 (2010); Gelczuk et al., J. Cryst. Growth 310, 3014 (2008).
[4] Affouda, Tomasulo et al., Appl. Phys. Lett. 110, 181107 (2017).

p-n junctions are at the heart of today’s semiconductor industry and are the basic building blocks of numerous electronic and optoelectronic devices. Properties of conventional semiconductor p-n junctions are well defined by the type of materials and doping during fabrication and is difficult to tune them afterwards. Van der Waals materials have made it possible to realize atomic thickness p-n junctions. Due to the reduced screening in the out of plane direction, 2D materials and their heterostructures with other dimensional materials offer a unique platform with greatly tunable optoelectronic properties. In this study, we demonstrate how engineering the surrounding dielectric environment would help a p-n junction to improve the photoresponse to a great extent. A silicon-monolayer MoS₂ heterostructure was fabricated as a model system and the effects of surrounding dielectric environment on the electrical transport and photoresponse of the heterostructure was studied. We report nearly three order magnitude enhancement of photoresponse in the presence of higher dielectric environment. With increasing dielectric constant of the sorrounding environment, band gap reduces in two dimensional materials changing the band alignment of the heterostructure, photogenerated carriers become easily separable and scattering of charge carriers reduces due to the screening of long range coulomb impurities contributing towards the photoresponse enhancement. Our findings lay a general pathway of improving the efficiency of 2D materials based photodetectors through dielectric engineering.

Diamond semiconductors, owing to their wide bandgap of 5.47 eV and superior properties compared to SiC and GaN, can be used to develop high-power devices. However, the inch-scaled diamond wafer is essential. Recently, we have demonstrated high-quality one-inch heteroepitaxial diamond wafer (Kenzan Diamond®) with X-ray rocking curve (XRC) FWHM of 113.4 arcsec, 234.0 arcsec for (004) and (311) diffractions and threading dislocation density of 1.4x10⁷ cm^-2 [1]. On the Kenzan Diamond, we have demonstrated field-effect transistors (FETs) with the highest available output power density of 345 MW/cm² [2]. So far, the Kenzan Diamonds were grown on Ir buffer layer/ just-oriented (11-20) sapphire substrate.
In this study, we performed diamond heteroepitaxial growth on Ir buffer layer/ misorientated (11-20) sapphire substrate. The heteroepitaxial diamond layer shows XRC FWHM of 98.35 for (004) and 175.3 arcsec for (311) diffractions. These values are the lowest and show the highest quality in heteroepitaxial diamonds. These results revealed that step-flow growth occurs on the misorientated surface and the misorientated layer structure releases the internal stress caused by the differences in coefficient of thermal expansion among diamond, Ir, and sapphire layers.
[1] S.-W. Kim et al., Appl. Phys. Lett. 117, 202102 (2020).
[2] N. C. Saha et al., IEEE Electron Device Lett, 41, 1066 (2020).

Since the first half of the 20th century, intercalation property of two-dimensional materials has been investigated energetically. Layered clay minerals such as montmorillonite are the well-known example, and intercalation of alkylammonium ion by exchange of the interlayer cation has been known to absorb hazardous molecules such as aromatic hydrocarbons. However, ideal functionalization has yet been achieved despite of abundance of the clay minerals. One of the biggest problems is difficulty in observation of the nanostructure of the interlayer space, whose height is merely around 1 nm. It is true that the gallery height is able to be measured by X-ray diffraction, but detail morphology of the intercalated molecules is not revealed easily by experiment. In this point of view, molecular dynamic (MD) simulation is a powerful tool to investigated the nanostructure of the organoclay and its intercalation property because the MD simulation directly visualized the nanostructure. Thus, in the present study, we performed the molecular dynamics simulation on montmorillonite modified by octadecyltrimethylammmonium ion (Mont-C₁₈) and the adsorption of anthracene (Ph₃) by using molecular dynamics simulation.
The gallery height of Mont-C₁₈ obtained by the MD simulation is 11 Å, which is consistent with that reported experimentally. C₁₈ ions are oriented horizontally and formed so-called pseudotrimolecular layer. The intercalation of Ph₃ induces vertical orientation of the C₁₈ ions, which was revealed by atom distribution analyses, and the value of the gallery height is increased linearly along with the number of the intercalated Ph₃ molecule. Corresponding to the orientation change of the C₁₈ ions, the Ph₃ molecules are oriented vertically against the Mont layer. By stepwise intercalation simulations, detail population changes of the Ph₃ molecules are revealed based on the molecular axis.
Atom distribution and orientation angle analyses reveals the detail swelling mechanisms of Mont-C₁₈: The intercalated anthracene molecules push out the C₁₈ ions, resulting in the orientation changes from the horizontal to the vertical. The Ph₃ molecules are correspondingly localized on the Mont layer with its molecular axis vertical. The latter result is consistent with that suggested experimentally, where weak interaction of the H atoms of the Ph₃ molecule with the Mont layer may be significant. Thus, it is found that the interlayer of the Mont-C₁₈ is highly inhomogeneous in the presence of the Ph₃ molecules even though the gallery height is only 11-21.6 Å, depending on the number of the intercalated Ph₃ molecules. In addition, the orientation of the Ph₃ molecules is parallel to each other as the number is increased. Because excimer emission is observed in this nanocomposite experimentally, it is strongly suggested that the localization of the Ph₃ molecules on the Mont layer is the key for the characteristic photophysical behavior.
We also concluded that the continuous increase in the gallery height originates from soft response of the C₁₈ ions to the intercalation. In other words, the swelling of the interlayer is ascribed not to the volume of the Ph₃ molecules but to the orientation changes in the C₁₈ ions. Summarizing the above results, it is concluded that Å-order inhomogeneity is introduced in interlayer by the intercalation of the Ph₃ molecules, which is significant to utilize the two-dimensional nanospace for ideal materials design.

In this work, we fabricated the different number of IZO interlayer into Al₂O_3-x based transparent resistive switching memory on a transparent ITO deposited substrate at room temperature. Al₂O₃/IZO heterostructure transparent memory has sufficiently high transmittance in the visible region. Also, Al₂O_3-x/IZO heterostructure transparent memory has improvement results in lower electroforming voltages as the number of IZO interlayer was increased, as compared to ITO/pure Al₂O_3-x/IZO transparent memory. At this time, the transparent memory with 2-IZO interlayer showed a reduction in forming voltage of more than 37% compared to ITO/pure Al₂O_3-x/IZO memory. The fabricated transparent memory exhibit typical bipolar resistive switching behavior regardless of the number of IZO interlayer. Additionally, a flexible Al₂O_3-x/IZO heterostructure memory was fabricated using the same process on ITO coated PET. The fabricated flexible transparent memory also maintained resistive switching characteristics and stable switching properties during bending.

In this work we theoretically analyze the collective modes of a heterostructure built from materials with different pairing symmetries. In particular, we focus on the regime where the bulk superconductor has a higher transition temperature and conventional s-wave pairing, while the proximitized system has a lower-temperature transition into an unconventional pairing phase. We argue that, even when both pairing symmetries are topologically trivial, there ought to be signatures of the unconventional pairing in the collective mode spectrum of the joint heterostructure. We conclude by discussing relevant experimental probes and how they relate to the intrinsic pairing interactions in the materials.

2d materials offer great advantages as building blocks for the next generation of technological devices, due to their high-quality, versatility, and the ability to create heterostructures of countless combinations, each exhibiting unique properties. Despite great strides made in device fabrication over the past two decades, some aspects are still missing from the 2d material toolbox, for example being able to heavily charge dope 2d materials locally and cleanly. The ability to dope 2d materials uniformly and non-destructively is highly desirable for both exploring material properties and developing state-of-the-art nanoscale devices. In this work, we characterize the ability of the layered Mott insulator alpha-ruthenium(III) chloride (\arucl) to strongly hole dope graphene. We present that \arucl\ not only yields a large charge transfer in carbon-based heterostructures, but also that it does so cleanly and uniformly with quantifiable homogeneity. We also find that while the charge transfer is robust for any thickness of \arucl\ down to the monolayer limit, and can extend out of plane to multi-layer graphene and through insulating spacer layers.

In recent years, Electron Diffraction, and especially the 4D-STEM [1] is growingly becoming a routine part of structural characterizations of materials at the nano-scale. Its un-matched spatial resolution (down to sub-nm) enables the exploration of local variations within a sample, which alternatively is averaged over the entire irradiated sampled area, when explored, for example, by x-rays. As often shown in electron-microscope, samples are often heterogeneous, and consequently their local properties, which then reflect on the average behavior of the material, composite, or device. Besides morphology and composition, the local structural order can vary, especially in evolving systems. In this study, we explore how far we can take electron diffraction when the interest is in the evolution of materials.
We challenge ourselves with mapping the local structure in a composite of crystalline Ni and amorphous Zr-Cu-Ni-Al Bulk Metallic Glass (BMG) was fused into a composite via hot-rolling [2]. Using a fast camera looking through a fluorescence screen on which diffraction patterns are observed, we use scanning precession electron diffraction to captured 4D-STEM data with 3 nm step size from a FIB-fabricated Ni/BMG/Ni laminate. Using an automated data reduction and analysis pipelines, including distortion correction, auto-masking, azimuthal-integration, Fourier-transformation, and least-square function fitting, to get electron Pair Distribution Function (ePDF) and a set of structurally-meaningful features, which we generalize as Quantities of Interest (QoI). Since we use both the 2D diffraction patterns and the 1D-ePDF’s, we generalize the concept of mapping structurally-significant QoI’s derived from a spatially-correlated diffraction pattern as scanning nano-structure electron microscopy (SNEM).
We use SNEM to generate QoI-based maps of, for example, the virtual-dark-field (VDF) from the 2D diffraction patterns, and the peak position, peak-width, and the goodness of fit to a structural model from the reduced ePDF’s. We find clear variations in structural features across the Ni/BMG interface, as well as within the BGM itself, which indicate the presence of nano-crystalline inclusions with higher structural correlation length, and evidence for variations in the degree of clustering of alike atoms (e.g., Zr-Zr) in the BMG, which indicate a varying short-range chemical order. We could also follow trends in compositional variations using the average interatomic bond-distance (as also verified with atomic fraction EELS maps) extract the distribution of atomic pairs, exposed the BMG/Ni inter-diffusion front, and estimate the effective local structure of the nano-crystalline inclusion within the BMG as an FCC structure.
Combining these results with atomic composition EELS maps, we learn that nano-crystalline inclusions within the BMG are located in Cu-deficient regions. We also learn that the formation of Ni/BMG composite via hot-rolling, which is often found to be challenging due to the excessive formation of nano-crystallites at the Ni/BMG interface has succeeded due to the metallic Ni being amorphized instead of [BMG--Ni] crystallites being formed. These results are an example of the richness SNEM can provide, and with the assistance of automated (and in the future - autonomated) pipelines, bring us closer to exposing (and potentially controlling) the history of heterogeneity in evolving systems.


[1] Xiaoke Mu, Andrey Mazilkin, Christian Sprau, Alexander Colsmann, Christian Kübel, (2019). Microscopy. 3554, 301
[2] Sina Shahrezaei, Douglas C. Hofmann, Suveen N. Mathaudhu (2019). JOM. 71, 2

SrVO₃ (SVO) is a conductive perovskite oxide with an electrical conductivity of 31µohm.cm and optical transparency (70 %) in the visible region [1]. Its lattice constant a = 3.8420 Å is conducive to integrating perovskite oxides on STO substrates for electro-optic applications. The growth window in pulsed laser deposition (PLD) (10^-6mbar and below) of SVO is relatively narrow to obtain the desired electrical and optical characteristics [2]. However, the integration of ferroelectric oxides like BaTiO₃ (BTO) requires a high deposition pressure of 5x10^-3 mbar and above [3].
To epitaxially integrate BTO on STO via SVO layer, a dual pressure growth scheme is developed to obtain switchable out-of-plane polarized BTO film in tandem. Our XRD measurements and in-situ RHEED measurements confirm the epitaxial nature of the BTO on SVO/STO. The spotty RHEED pattern of the BTO surface indicates BTO grows on SVO via a Volmer-Weber growth mode. Both BTO and SVO films exhibit a sub-nanometer roughness. The SVO film resistivity (Vander-paw method) is 23.2µohm.cm, comparable to that of MBE-grown films. The BTO integrated via SVO/STO has a good crystal quality, as seen from symmetric ω-scan FWHM of 0.31°. The lattice parameters extracted from out-of-plane (00l) and in-plane (h0l) RSM scans confirm the out-of-plane orientation of the BTO film. The piezo force microscopy measurements show the switchable nature of polarization in the BTO film. A large tetragonality (c/a-1 %) of >2 % (vs. 1 % for a single crystal BTO) is observed in our BTO films and is dependent on deposition conditions. The underlying mechanism will be discussed in detail at the conference.
References
<sup>1.M.Mirjolet et.al,Adv.Func.Mater.29,1904238 (2019),
2. M.Mirjolet et.al, Adv. Func. Mater. 29, 1808432(2019),
3. J Lyu et.al., Applied Mat. Interface, 10, 25529-25535, (2018).
4. Jarrett et.al, Adv.Mat.,25,3578-3582,(2013).</sup>

Advances in telecommunication technology has led to demand for faster operating frequencies and wider usable frequency ranges. The development of 5G and 6G communication technologies will, in particular, require devices capable of operating at frequencies beyond 90 GHz and up to 300 GHz. Currently, the two semiconductor technologies most used for RF devices are silicon complementary metal-oxide-semiconductor (CMOS) field-effect transistors (FETs) and transistors based on III-V compound semiconductors (primarily GaAs). Silicon transistors are capable of high levels of integration at low cost, but they suffer from poor linearity and noise performance at frequencies above 20 GHz. Devices based on III-V compounds can be used to build systems operating at frequencies beyond 20 GHz, but cannot be integrated directly with digital system in the same chip.
High-semiconducting-purity CNT arrays with suitable choice of array density and tube diameter should allow compatible fabrication of ultra-scaled digital CMOS FETs and high-performance RF transistors on the same substrate. Therefore, CNT FETs should make a better platform for SoC applications than silicon and III-V compound semiconductors-based technologies.
The performance of CNT transistors has been improved due to the developments in material preparation and device fabrication. FETs with the current gain cut-off frequency (f_T) and the power gain cut-off frequency (f_MAX) both over 100 GHz have been demonstrated, far behind theoretical predictions, because of lack of well-aligned semiconducting CNT arrays. Ideally, the CNT material for RF FETs is arrays with density ranged from 100 to 200 CNTs μm^-1, high semiconducting purity and as a high carrier mobility as possible. The most promising RF FETs have been built using arrays prepared through the floating evaporative self-assembly (FESA) method based on solution-derived CNTs, but their speeds are still limited by the relatively low density (<70 CNTs μm^-1) and low mobility (230 cm²V^-1s^-1).
Here, we report aligned CNT arrays that offer high density (120 CNTs μm^-1), high semiconducting purity (at least 99.99%), carrier mobility (up to 1580 cm² V^-1 s^-1) and a saturation velocity of up to 3.0×10⁷ cm s^-1. We use a double-dispersion sorting and a binary liquid interface-confined self-assembly (BLIS) procedure, which allows for assembly of aligned arrays on both insulating or semiconducting substrates, such as silicon, glass and quartz. These processes for RF FETs are designed to be fully compatible with the fabrication digital CMOS CNT FETs, for SoC applications. FETs based on these CNT arrays exhibit high driving current, transconductance and f_T/f_MAX on both quartz and Si/SiO₂ substrates. In particular, FETs with a 50 nm gate length on Si/SiO₂ offer high d.c. performance with on-state current of 1.92 mA/µm and peak transconductance of 1.40 mS/μm at a bias of -0.9 V, and show high a.c. performance with f_T/f_MAX of 540 GHz and 306 GHz, respectively, and we show that f_T can be pushed beyond 1 THz when scaled to gate lengths of sub-28 nm. RF amplifiers made using 50 nm length FETs with multi-finger structure exhibit high power gain (23.2 dB) and inherent linearity (31.2 dBm OIP3) in frequencies of at least up to the K band (18 GHz).

Sb₂Te₃ is well known as a phase-change and thermoelectric material and has recently attracted significant attention as a topological insulator (TI). TIs possess non-trivial electronic structures originating from the strong spin-orbit interaction, resulting in unusual properties, such as the presence of helical spin currents on the surface and formation of the Dirac cone. These properties are expected to enhance spin pumping. However, the effects of atomic ordering in TI films on spin relaxation have not yet been investigated. In this work, the spin-pumping effect in CoFeB/ Sb₂Te₃ bilayers was studied using ferromagnetic resonance (FMR) at 300 K for crystalline and amorphous Sb₂Te₃. FMR signal width enhancement was observed for the highly oriented crystalline Sb₂Te₃ film at thicknesses less than 7 nm compared with the amorphous counterpart. In contrast, the FMR signal widths of the amorphous sample and Pt reference were independent of the layer thickness. In particular, the enhancement of the linewidth was most remarkable at 4-nm thickness for the crystalline Sb₂Te₃ samples; this feature was not observed for amorphous Sb₂Te₃ samples. The significant increase in linewidth suggests more efficient spin injection from the ferromagnetic CoFeB layer into crystalline Sb₂Te₃ than into amorphous Sb₂Te₃. Furthermore, the stark contrast between the spin dynamics of the amorphous and crystalline Sb₂Te₃ films supports the idea that the topological phase protected by crystallographic symmetry plays a key role in spin pumping. Additionally, for the effective magnetization of Sb₂Te₃ samples, a change of the magnetic anisotropy direction depending on Sb₂Te₃ thickness was observed, implying the possibility of interactions between topological surface state in crystalline Sb₂Te₃ and ferromagnetic CoFeB films. These findings enhance understanding of the spin pumping effect between topological insulators and ferromagnetic materials and strongly suggest that it is possible to modulate the amount of injected spin by controlling the crystal structure, which provides a new perspective on spin conduction control using phase-change phenomena in conventional spintronics research. This could enable the realization of spintronic devices based on phase-change topological materials.[1]
[1] M.Morota et al., Phys. Status Solidi Rapid Res. Lett. (2021) DOI: 10.1002/pssr.202100247

Graphene is one of the promising materials for realizing a wide range of applications from its unique electrical, mechanical, and chemical characteristics. We have studied a direct precipitation method of graphene to improve its productivity with avoiding the transfer process. The direct precipitation method is promising, although the characteristics of the precipitated graphene are still inferior to those of the CVD-grown graphene. To improve the quality, high temperature growth is attractive but the grain boundary of the catalyst brings another problem. It is because the carbon atoms easily diffuse through the boundaries of the catalyst even at low temperature, and severely deteriorates the precipitated graphene. Therefore, the crystallization of the catalyst without grains is an important issue in the study of graphene growth. The crystallization is beneficial to obtain a large graphene domain too because the superior crystallinity of the catalyst is good for aligning the orientation of the graphene islands and, subsequently, an ideal stitching of the graphene islands are expected. In this presentation, the crystallization of the Ni catalyst is firstly studied on the sapphire substrate. Then, the crystallized Ni catalyst is used to directly precipitate multilayer graphene.
A 100 nm-thick Ni (111) layer was grown on a c-plane sapphire by MBE. Then, 300 nm-thick Ni layer was deposited using electron-beam deposition. The samples were annealed at 1000^oC for 30min to crystallize it under hydrogen at atmospheric pressure. Then, the temperature was cooled down at a rate of 3^oC/min. After the formation of the Ni crystallized catalyst, the sample was coated by nanodiamond, and heated at 900^oC for 30min in a vacuum (10⁻⁴–10⁻³ Pa) using an infrared lamp apparatus. Then, the temperature was cooled down at a rate of 3^oC/min. The catalyst was removed using a diluted FeCl₃ solution to directly observe the graphene on the substrate.
Almost whole surface of the sample was successfully crystallized by using the MBE layer as a seed, and a perfectly smooth and flat surface was obtained on it. It is useful to deposit an additional Ni amorphous layer on the MBE-grown layer because the amorphous layer is effective to suppress the dewetting of the Ni layer even at a high temperature.
An extremely flat surface with several growth islands was obtained on the precipitated multilayer graphene. The in-plane X-ray diffraction spectra shows an excellent crystallinity of the graphene from its quite narrow FWHM of 0.25^o, which is the smallest FWHM yet reported for a graphene layer. The graphene [11] orientation was precisely aligned to the sapphire [1-100] axis. The Raman spectra indicates the precipitated graphene is multilayer. The Raman D/G mapping measurement with no associated increase also shows that their islands were smoothly connected without forming defects. In addition, no wrinkle was observed on the surface, which possibly came from the effective release of the contraction stress with gently slipping the layer with sandwiching both sides.
Acknowledgements: This work was supported in part by JSPS KAKENHI Grant Numbers 25000011, 15H03558, 26105002, and 2660089.

Recently, the multi-component metallic thin films having the phase-separated microstructure have been growing rapidly in recent years due to the improved mechanical properties. The size effect of the length scale of the phase-separated region with the character of the interface defines the behavior of the thin films towards the mechanical properties. A comparative study has been carried out on the evolution of the microstructure of the co-deposition of Al-Si thin films at room temperature processed by DC magnetron sputtering by using high purity Al and Si targets. During the deposition, the Si concentration of the as-deposited films is found to increase by the expense of Al content by tuning the Si and Al target powers. The chemical compositions of the films have been evaluated using energy-dispersive X-ray spectroscopy, whereas the microstructure has been examined by high-resolution as well as scanning transmission electron microscopy. The film having the lowest Si concentration (Al₈₀Si₂₀) consists of the phase-separated microstructure with the long columnar grains of Al with a randomly distributed Si phase. On the other hand, the film with high Si concentration (Al₅₅Si₄₅) comprises the flake-like structure of Al with the amorphous Si phase. The mechanism of the transition in the morphology of the films by varying the Si target power will be discussed based on deposition rate as well as the inter-diffusion length of the diffusing adatoms of the Al and Si during the deposition process.

Two-dimensional van der Waals heterostructures have attracted increasing interest, owing to the combined benefits of their constituents. These hybrid nanostructures can be realized via epitaxial growth, which offers the prospect of achieving ultraclean interfaces with strong electronic and optical coupling between layers. Here, the epitaxial growth of a continuous molybdenum disulfide (MoS₂) film on large-area graphene, which was directly grown on a sapphire substrate, is reported. Interestingly, the grain size of MoS₂ grown on graphene increases, whereas that of MoS₂ grown on SiO₂ decreases with an increasing amount of hydrogen in the chemical vapor deposition reactor. In addition, to achieve the same quality, MoS₂ grown on graphene requires a much lower growth temperature (400 °C) than that grown on SiO₂ (580 °C). The MoS₂/graphene heterostructure that was epitaxially grown on a transparent platform was investigated to explore its photosensing properties and was found to exhibit inverse photoresponse with highly uniform photoresponsivity in the photodetector pixels fabricated across a full wafer. The MoS₂/graphene heterostructure exhibited ultrahigh photoresponsivity (4.3 × 10⁴ A W⁻¹) upon exposure to visible light of a wide range of wavelengths, confirming the growth of a high-quality MoS₂/graphene heterostructure with a clean interface.

Remote epitaxy, via so-called 2D material-based layer transfer (2DLT) has enabled a wide range of materials to be successfully grown as thin, free-standing layers that can be readily integrated into devices. Such method can be applied in the area of electrochemistry, where we can improve and overcome the limitations that liquid-based Li ion batteries have suffered over the past several decades. While liquid electrolytes have been mainstream owing to its high ionic conductivity and good wetting property with electrode surfaces, its fundamental limitations are set by low thermal, electrochemical stability, low energy density, and poor safety due to dendritic formation.

In particular, formation of Li dendrites penetrating through liquid electrolytes are identified as the main cause for explosions via thermal runaway, which also reduces the lifecycle of the battery. To solve this issue, solid-state batteries have been proposed as an alternative to conventional lithium batteries, which promise significantly reduced possibility of dendrite formations without sacrificing any other properties. However, high-performance thin single-crystalline solid-state electrolytes (SSE) have not yet been demonstrated since Li-containing ionic compound wafers are not commercially available in a single-crystalline form, as well as the difficulty to fabricate thin film SSE single-crystals, necessary for compact design of batteries with high volumetric energy and power density.

Here, we will demonstrate the use of graphene coated substrates for remote epitaxy of LiLaTiO₃ (LLTO) as the electrolyte for solid-state battery applications. We show comparable performance of our ultra-thin film battery to conventional lithium batteries and discuss overall improvements we are able to achieve.

Molecular nanolayers (MNLs) at inorganic interfaces are known to lead to unusual enhancements in interfacial fracture energy, thermal and electrical transport. Here, we demonstrate viscoelastic damping bandgaps in multilayered interfaces of MNL-bonded Au nanolayers. Molecular dynamics simulations of Au/octane dithiol/Au multilayers reveal high-damping-loss frequency bands in the 33 ≤ ν ≤ 77 GHz and 278 ≤ ν ≤ 833 GHz regions separated by a low-loss bandgap. The high-damping-losses are up to 31-fold higher than that in individual components in the bulk forms, and the low-losses range from ~ 40% to 120% of the bulk values. The viscoelastic bandgap scales with the Au-MNL interfacial bonding strength and density, and MNL coverage indicating that it is an interface effect. These findings suggest a variety of possibilities for accessing and tuning novel dynamic mechanical responses in nanocomposites, devices and systems with significant inorganic-organic interface fractions for applications.

SnTe is one of the first materials in the class of topological crystalline insulators (TCIs) to be predicted and studied. In this work, we synthesize thin SnTe films (10 to 30 nm thick) patterned with a regular array of square holes on a SrTiO₃ substrate and compare quantum transport with that measured in unpatterned thin films. The weak localization/antilocalization (WL/WAL) in films is revealed by fitting the magnetoconductivity measurements with the Hikami-Larkin-Nagaoka (HLN) equation. The results show a mixture of WL and WAL in both films, indicating a partial electron spin-momentum locked state. The presence of square holes in the SnTe films suppresses conventional conduction at higher temperature, leading to a stronger WAL effect, and limits the coherence length to the hole size at low temperatures.