Symposium EL12—From Molecules to Emerging Molecular Devices—Beyond Electronic Transport

Break junctions are a popular method to characterize the electrical properties of single moleucles [1]. However, the behaviour of a molecule in a junction is complicated and therefore difficult to assess experimentally. For example, the end to end single-molecule (fully stretched) configuration may not be the most probable one, while this is the benchmark in the comparison with theoretical calculations. So, the questions “What are the pathways for charge transport through a molecule and what is the conductance of the fully stretched molecule” can still not be answered in all cases. In this talk, I will discuss possible causes for the observed dispersion in the conductance histograms and show that by using unsupervised clustering methods, [2] preferably in combination with clever chemical engineering of molecule families, conduction pathways can be identified and the end-to-end single-molecule conduction can be established [3-6].

References
[1] Single-molecule quantum-transport phenomena in break junctions, P. Gehring, J.M. Thijssen and H.S.J. van der Zant, Nature Reviews Physics 1 (2019) 381 – 396 (DOI: 10.1038/s42254-019-0055-1).
[2] A reference-free clustering method for the analysis of molecular break-junction measurements, D. Cabosart, M. El Abbassi, D. Stefani, R. Frisenda, M. Calame, H.S.J. van der Zant and M. L. Perrin, Applied Physics Letters 114 (2019) 143102 (DOI: 10.1063/1.5089198).
[3] Unravelling the conductance path through single-porphyrin junctions, M. El Abbassi, P. Zwick, A. Rates, D Stefani, A. Prescimone, M. Mayor, H.S.J. van der Zant and D. Dulić, Chem. Sci. 10 (2019) 8299 – 8305 (DOI: 10.1039/c9sc02497b).
[4] Intermolecular Effects on Tunneling through Acenes in Large-area and Single-molecule Junctions, Y. Liu, L. Ornago, M. Carlotti, Y. Ai, M. El Abbassi, S. Soni, A. Asyuda, M. Zharnikov, H.S.J. van der Zant and R.C. Chiechi, Phys. Chem. C 124 (2020) 22776 – 22783 (DOI: 10.1021/acs.jpcc.0c05781).
[5] Conformation-dependent charge transport through short peptides, D. Stefani, C. Guo, L. Ornago, D. Cabosart, M. El Abbassi, M. Sheves, D. Cahen and H.S.J. van der Zant, Nanoscale 13 (2021) 3002 – 3009 (DOI: 10.1039/D0NR08556A).
[6] Benchmark study of alkane molecular chains, F.H. van Veen, L. Ornago, H.S.J. van der Zant and M. El Abbassi, The Journal of Physical Chemistry C 126 (2022) 8801 − 8806 (DOI: 10.1021/acs.jpcc.1c09684).

Air and thermally stable organic radical molecules are ideal candidates for molecular spintronic devices. Although prior work has considered a wide variety of chemical structures for spintronics applications, key challenges remain in understanding the structure-property relationships in organic radical molecules at the molecular level. In this work, we study the charge transport properties of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) radical containing non-conjugated molecules using single molecule conductance experiments and semi-empirical quantum chemical simulations. Our results show that the TEMPO pendant groups promote the molecular charge transport compared to the quenched TEMPOH and neutral benzyl pendant groups. Furthermore, the molecular conductance of TEMPO-containing molecules decays with increasing length of molecular backbones similar to non-conjugated alkane chains, but the molecular conductance remains at larger values compared to alkane chains. Temperature-dependent single molecule experiments show that tunneling is the dominant charge transport mechanism in TEMPO-containing molecules. Experimental results are complemented by molecular simulations, which reveal the importance of molecular conformation on charge transport. Results from molecular simulations further show that TEMPO radicals interact with gold metal electrodes near the interface to facilitate a stabilized high-conductance configuration. Overall, our work directly reveals enhanced charge transport in non-conjugated molecules by incorporating open-shell radicals, which will help to inform the design of new materials for molecular electronic devices.

Incorporation of paramagnetic and charged molecules into single molecule circuits can increase the scope of observable phenomena due to new degrees of freedom in such molecules. Here I discuss my group’s recent advances in establishing chemical design rules for forming stable junctions with coordination complexes containing transition metal atoms, charged linkers and organic paramagnetic compounds. We use Scanning Tunneling Microscope Break Junction (STMBJ), density functional theory (DFT) and non-equilibrium Green’s Function methods to determine conductance, geometry and evolution of molecular junctions. We successfully form and measure the conductance of organic diradicaloid compounds with installed thioanisole linkers. In contrast, the formation of junctions with ex situ synthesized paramagnetic coordination complexes containing transition metal atoms is complicated due to disassembly of these complexes on gold. Instead, we find that transition metal centers can be reliably wired into single molecule junctions by coordinating in situ to anionic linkers during junction elongation. Through environmental control, we can tune the charge on anionic molecules to affect formation and transport properties, including quantum interference effects. In situ control and assembly of molecular junctions using charged moieties presents a new approach for creating molecular circuits with non-trivial electronic and spin functionality.

Over the past decade, there has been tremendous progress in the measurement, modeling and understanding of structure-function relationships in single molecule circuits. Experimental techniques for reliable and reproducible single molecule junction measurements have led, in part, to this progress. In particular, the scanning tunneling microscope-based break-junction technique has enabled rapid, sequential measurement of large numbers of nanoscale junctions allowing a statistical analysis to readily distinguish reproducible characteristics. Although the break-junction technique is mostly used to measure electronic properties of single-molecule circuits, in this talk, I will demonstrate its versatile uses to understand both physical and chemical phenomena with single-molecule precision. In this talk, I will present results from a recent work where we demonstrate that molecular wires can behave as one-dimensional topological insulators. We focus on a family of oligophenylene-bridged bis(triarylamines) with tunable and stable (mono-/di-)radicaloid character. These wires can undergo one- and two-electron chemical oxidations to the corresponding monocation and dication, respectively. The oxidized wires exhibit high reversed conductance decay with increasing length, consistent with the expectation for the Su-Schrieffer-Heeger-type one-dimensional (1D) topological insulators. Importantly, we show that for one length, the dication displays a significantly high conductance greater reaching beyond 0.1 G₀ (2e²/h, the conductance quantum).

Creating single molecule-metal junctions containing metal ions or organic radicals is a promising route to achieving functional molecular devices. Here I discuss our recent advances in understanding formation and evolution of molecular junctions formed with organometallic molecules and organic radicals. Our work indicates that the atomic structure of the electrodes can influence charge transfer at the metal-molecule interface and affect junction evolution characteristics. With organic radicals, controlling charge transfer is critical to preserving radical properties. We measure the conductance of indenoindenodibenzothiophene (IIDBT) diradicaloid, an organic molecule with a stable singlet diradical-like character in its ground state, using Scanning Tunneling Microscope Break Junction technique. We incorporate thioanisole linkers into the molecule to decouple the electronic state from the electrode and to anchor it in place. We find reproducible conductance signatures using these molecules in ambient conditions in organic solvents. A combination of experimental and theoretical work using DFT suggests this family of stable diradicaloid organic molecules is a promising choice for studying and understanding electron transport through diradicaloid molecule-metal junctions.

Recent years have seen a remarkable progress in the field of organic electronics with the in-plane charge carrier mobility routinely surpassing 10 cm²/Vs in the best-performing materials. However, the miniaturization of organic semiconductor devices to achieve sub-micrometer active channel lengths and high switching frequencies is still limited by the contact resistance between metal electrodes and the organic semiconductor material. Efficiently controlling the contact and access resistance is, therefore, of crucial importance in pursuit of developing efficient electronic devices based on organic materials.
Conventional approaches for achieving low contact resistance usually focus on interface engineering to achieve better alignment of energy levels. However, another significant contributor to high contact resistance is the out-of-plane mobility. Despite its importance, the out-of-plane mobility remains an often-overlooked parameter mainly due to the lack of available measurement techniques.
Herein, we report a novel method for evaluating the out-of-plane mobility and contact resistance based on conductive-probe atomic force microscopy (C-AFM). In this study, we prepared multi-layered single crystal films of the molecular semiconductor C8-DNTT-C8 by a solution shearing technique on electrically conducting substrates and investigated them using C-AFM. By using scanning probe microscopy to map the local changes in the out-of-plane electrical current we gain a close-up view of the out-of-plane conductivity and access resistance at an unprecedented molecular length scale. This study opens a new pathway for gaining a better understanding of structure-property relationships and enabling a more accurate engineering of materials and interfaces in organic semiconductor devices.

Ladder-type constitution strongly enhances the chemical stability and yields well-defined physical properties that enables the fundamental studies of charge transport in multi-state, reversibly switchable aniline derivatives. Herein, we report the change in charge transport properties of a conjugated ladder-type cyclohexadiene-1,4-diimine directive at various protonation, lithiation and oxidation states. Our results demonstrate a 10-fold conductance increase from the reduced benzenoid structure to the oxidized quinonoid structure. Furthermore, acid/lithium salt addition renders two-step protonation/lithiation of the quinonoid molecule to a dicationic form in resonance with an air-stable open-shell diradical state. The fully protonated/lithiated state displays a conductance level 15 times higher than the neutral quinonoid state. Surprisingly, the charged quinonoid state also exhibits the possibility of interchain charge transport, which is not observed in the neutral state. A core-anchor group type and anchor group-anchor group type of dimerization are found to contribute to the intermolecular charge transport via DFT simulations(consider to remove such detail in abstract). Beside the chemically induced conductance changes, a stable and reversible conductance switch of over two orders of magnitude is achieved in-situ by electrochemical gating. Overall, this work highlights a highly robust, multi-state, single molecule switch, in which fully fused ladder-type constitution is incorporated to advance the stability, programmability, and efficiency of the single-molecule devices.

The development of devices based on single-molecule junctions requires not only robust control over molecular self-assembly but integration into structures that can interface with the macroscopic world. Accomplishing this task requires a manufacturing technique that simultaneously offers the reproducibility and accuracy of photolithography as well as the nanoscale control offered by solution phase self-assembly. In order to unify top-down and bottom-up approaches to nanoengineering, we are developing physical supercritical fluid deposition. This technique leverages the unique solubility properties of supercritical fluids to deposit molecules without the need for in-situ chemical reactions. The process relies on a peak in the isobaric saturation solubility with respect to temperature allowing thin films to be formed via precipitation onto heated substrates. The location of material deposition can be accomplished by controlling the local temperature on the substrate via simple joule heating. The resistive traces used for joule heating can be created via photolithography, allowing us to direct the solution phase self assembly with the accuracy of photolithography. Progress towards improving the resolution limit of the technique will be discussed as will the mechanisms of self-assembly and their effect on the nanoscale morphology of polymeric materials.

The development of next-generation organic electronic materials critically relies on understanding structure-function relationships in conjugated polymers. However, a major challenge lies in accessing the vast chemical space encompassed by synthetic organics while efficiently managing the large synthetic workload to survey new materials. Here, we use automated synthesis to overcome synthetic bottlenecks hindering access to the large chemical space for organic electronics. A library of terphenyl derivatives with different side chain compositions and anchors was prepared using a custom synthesis instrument for automated iterative Suzuki coupling. The role of side chain chemistry on the charge transport properties of conjugated oligomers was then systematically studied using single-molecule charge transport experiments. Our results show an unexpected concentration-dependent bimodal conductance in terphenyl molecular junctions with long alkyl side chains. We found that high conductance states arise due to surface adsorption and backbone planarization induced by the long side chains, which is confirmed by density functional theory simulations and a series of control experiments with asymmetric, planarized, and sterically hindered oligomers. Molecular junctions with oxygen-containing side chains are also investigated, showing high levels of conductance and reduced molecular adsorption behavior compared to alkyl side chains. Overall, our work highlights the role of side chain chemistry on charge transport and opens new avenues in combining automated synthesis with single molecule characterization to develop new materials for organic electronics.

When electrons are injected through a chiral molecule, the resulting current may become spin polarized. This effect, known as the chirality-induced spin-selectivity (CISS) effect, has been suggested to emerge due to the interplay between spin–orbit interactions and the chirality within the molecule. However, such explanations require unrealistically large values for the molecular spin–orbit interaction without any physical justification. Put simply, to date, the physical origin of the CISS effect is unknown.Here, we present the “spinterface mechanism” for the CISS effect, based on the interplay between spin–orbit interactions in the electrode, the chirality of the molecule (which induces a solenoid field), and spin-transfer torque at the molecule–electrode interface. The spinterface mechanism has already been shown to allow remarkable fit between theory and data for numerous experiment. We will provide new examples for such fits, further corroborating the mechanism. We will describe a set of experiments which can be used as a “smoking gun” for differentiating this mechanisms from other theoretical explanations. Finally, we will describe a spinterface mechanism for the CISS effect in photo-excited electrons scattered off a layer of chiral molecules.

Biomolecules such as proteins and peptides are potential candidates for building diverse functional electronic devices. Measuring and controlling electron transport (ETp) through these molecules, especially, as assemblies on solid substrates, i.e., part of an electronic device structure with solid electrodes, is a major goal of bio-molecular electronics. The electronic structure of the peptide/protein monolayer and/or its orientation in contact with an electrode can be directly related to the energy barrier and electrode/molecule coupling in the resulting junction.
Ultraviolet and X-ray Photoelectron Spectroscopies (UPS, XPS) with their high surface sensitivity have been used extensively to study the electronic and chemical structure of molecular thin films or monolayers on solid surfaces. Here we show how UPS and XPS measurements can provide a crucial information for understanding the mechanism(s) of ETp through biomolecule/Au electrode-containing junctions.
In the first example we show how the spatial position of a singe tryptophan “dopant” in a linear oligo-alanine chain may significantly influence ETp across a self-assembled monolayer of these peptides between gold contacts.¹ In the second example we explore the effect of the side-chain of the peptide (2Ala vs 2Trp) on its electronic properties.²
In the two next examples, the bacterial electron transfer protein Azurin (Az) ³ and a mutant (N42C Az)⁴ are studied on Au substrates. We find that the ETp mechanism can be changed by changing just a linker (binding)³ or the orientation⁴ of the protein with respect to the Au substrate.
Finally, we demonstrate that the direction of current rectification and the conductance-voltage characteristics of the Photosystem 1 complex (PS1) can be controlled by its orientation on Au substrates.⁵


1. C. Guo et al, PNAS, 113, 2016, 10785-10790.
2. C. Guo et al, PCCP, 20, 2018, 6860-6867.
3. J. A. Fereiro et al, JACS, 140, 2018, 13317-13326.
4. J. A. Fereiro et al, JACS, 142, 2020, 19217-19225.
5. J. A. Fereiro et al, Small, 2022, submitted.

Due to its programmable nature and rapid modern-day production, DNA has seen and gained wide interest in being used in the advent of novel nano-scale devices, giving rise to the field of DNA nano-technology. One novel application, and of specific interest, is that of the use of DNA molecules as charge transfer mediums for molecular nano-electronic devices. For such devices to be of practical use, a thorough understanding of the electrical properties of the charge-transfer mechanics within the DNA molecule is required. Attempts at characterizing the electrical properties of individual DNA molecules have been previously performed, however, there still remains much disagreement among the scientific community regarding the classification of DNA molecules. Thus far, DNA has been found to be a conductor, a super-conductor, semiconductor, and an insulator. These conflicting results can be reconciled due the use of different setups during each experiment, ranging from differing methods of physical contact to the misplacement of the probe and or the method of probing the DNA molecules in question. In order to avoid such issues, a standard non-invasive and repeatable method is required to characterize the DNA molecules. To do so, in our research, we characterize individual hybridized DNA molecules attached, on both ends, to gold via thiol linkers using a mechanical non-contact method. Silicon-Gold substrates are prepared on which a DNA Self Assembled Monolayer is grown and later hybridized with Gold NP functionalized complementary DNA. The charge transfer through the DNA molecules is then measured via non-contact mechanical charge sensing by atomic force microscopy (AFM) [1] in vacuum at cryogenic temperatures. The charge transfer rate can be measured quantitatively by measuring the ratio fo the resonance frequency to the damping of the oscillating AFM cantilever [2]. We will present the experimental results measured for several short DNA sequences.

[1] Y. Miyahara, A. Roy-Gobeil and P. Grutter, Nanotechnology 28, 064001 (2017).
[2] A. Roy-Gobeil and P. Grutter, Nano Lett. 15, 2324 (2015).

Quantum computing is a rapidly emerging technology that has the potential to revolutionize computations unattainable with classical computers, and f-element systems have shown exceptional promise in quantum information processing. While progressing through the actinide series, the relativistic effects change the energetics of the 5f and 6d orbitals wherein control of a quantum bit (qubit) could be possible. For example, protactinium contains interesting characteristics and adopts the behavior of transition metals because of its near degeneracy of the 6d and 5f orbitals. This presentation outlines the potential for high-valent 5f¹ early actinide complexes to act as qubits by evaluating the atomic properties, including spin-orbit coupling, crystal electric field, and nuclear spin. This research is set to establish basic understanding of the interplay between the d and f orbitals thereby generating a set of features to enhance quantum coherence in actinide complexes.

Developing a fundamental understanding of the interactions between biogenic molecules and noble metal surfaces is important for guiding and advancing the development of biotic-abiotic interfaces, conductance measurements of biomolecule conductivity, and biosensing. Here, we use the Scanning Tunneling Microscope-based Break Junction (STMBJ) technique to measure the conductance and binding orientation of a single nucleobase, Adenine (Ade), and its derivatives on gold. We systematically study the conductance features of Ade and other structurally similar molecules that lack one or more of the candidate binding sites of Ade to assign individual molecular conductance features to specific metal-Ade binding configurations. Our results allow us to determine the binding orientations of Ade on gold unambiguously. To demonstrate the biosensing potential of these measurements, we show that we can sense and distinguish Ade from its biologically relevant variants, 6-methyladenine and 2'-deoxyadenosine. Finally, we apply these methods to determine the molecular conductance and binding configurations of other biologically functional molecules on gold, such as histidine and histamine. Our work establishes STMBJ as a powerful tool for characterizing the atomic structure of metal-organic interfaces and lays the foundation for biosensing on gold using single molecule conductance readout.

Vertical organic transistors (VOTs) are emerging electronic devices that can overcome the limitations of conventional organic thin film transistors (OTFTs) based on lateral architecture. Here, organic permeable base transistors (OPBTs) were formed with an oxide layer on the base electrode as a dielectric layer by using an electrochemical oxidation called anodization, and passivated by self-assembled monolayer (SAM) treatment after forming AlO_x on the aluminum base electrode. The VOTs with the electrochemical oxidation and molecular passivation are simply manufactured and the charge carrier transport is significantly improved. SAM treatment is a regularly ordered organic monolayer that is spontaneously formed on a solid surface. Since the properties of the surface can be controlled, it is used on the surface of a base insulator to improve the carrier transmission performance of organic transistors. In this study, the oxidation and passivation of permeable base electrode were systematically controlled. The SAM treatment can reduce the leakage current of the base electrode and improve the current transmittance between the emitter and the collector. In addition, we observed the specific behaviors of vertical OPBT depending on the applied potential correlations between the middle base electrode and the bottom collector electrode.

Understanding and manipulating quantum interference (QI) effects through chemical design and external control is important for the development of molecular-scale devices. QI results from electrons tunneling through multiple molecular orbitals (MOs) each with a distinct phase with respect to the electrode attachment points on the molecule. In this work, we use Scanning Tunneling Microscope Break Junction (STMBJ) single molecule conductance measurements and electronic transport calculations to explore the effects of chemical composition and substitution in pyrazine-based molecular junctions. We find that QI effects dominate observed transport characteristics of these molecules, resulting in distinct conductance peaks through the meta but not the para pathway. Critically, destructive interference through the para position of the pyrazine ring can be manipulated via substitution at the meta position. By varying the environment surrounding the junction, we can control the charge of the chemical substituents and manipulate the alignment of individual MOs with the Fermi energy to tune QI through the para channel in pyrazine-based junctions from destructive to constructive. This work demonstrates environmental control of molecular QI, opening the door to in situ tuning of QI effects in single molecule junctions.

Polyaniline (PANI) is of considerable interest due to its versatile chemical and electrochemical doping states. However, they suffer from chemical instabilities at acidic and oxidative conditions, which limits their applications in higher oxidative and protonated states. Herein, we reported a series of ladder-type PANI derivatives flanked with methylthiol groups on both ends as the model molecule for single-molecule conductance studies. The implementation of the ladder-type constitution prevents undesired hydrolysis and isomerization, rendering high stability of these molecules at different protonation and oxidation states. We studied the single-molecule charge transport properties of ladder-type PANI derivatives using the scanning tunneling microscope-break junction (STM-BJ) method. Our results demonstrate a 10-fold conductance increase from the reduced benzenoid structure to the oxidized quinonoid structure. Furthermore, leveraging the chemical stability that ladder-type constitution provides, acid/lithium salt addition renders two-stage protonation/lithiation of the quinonoid molecule to a dicationic form which shows open-shell diradical character in ambient conditions. The fully protonated/lithiated state displays a conductance level 15 times higher than the neutral quinonoid state. Surprisingly, intermolecular charge transport is also observed via the formation of stacked dimers assisted by pi-pi interaction. The experimental results are complemented by DFT calculations, which reveal smaller energy gaps and extended molecular conformations for higher protonated and lithiated states. Overall, this work highlights a highly robust, multi-state, single-molecule switch where a ladder-type constitution is incorporated into the PANI system to advance the stability, programmability, and efficiency of the single-molecule devices.

Phonons limit charge mobility in organic semiconductors by dynamically localizing charge carriers to a few molecules. The main synthetic strategy to overcome dynamic localization has been to use side chains to “damp out” phonon modes that limit charge mobility. Here we examine a different strategy, to introduce “large” small molecules that form extended crystals and reduce molecular motion. Tri(n-hexyl)silylethynyl benzodithiophene (BDT) trimer, has demonstrated one of the highest mobilities recorded (1.6 cm²/Vs). To understand why the BDT trimer performs so well, the phonons spectra, are computed using our DFT based open source Davis computational spectroscopy (DCS) workflow and verified using inelastic neutron scattering (INS) measurements. We separately developed a workflow called ElPh to compute the atom specific contribution to electron-phonon coupling, which enables us to quantitatively map which parts of the molecule are actively engaged in charge mobility reduction. This process offers understanding for why this material performs so well. The simulation process is transferrable to design of other high-mobility organic electronic materials.

Stable organic semiconductors (OSCs) with a high-spin ground state can profoundly impact emerging technologies such as organic magnetism, spintronics, and medical imaging. Over the last decade, a significant effort has been to design p-conjugated materials with unpaired spin centers. In this presentation, we report small molecules with a high spin ground state and large polyradical character with a small singlet-triplet energy gap, not observed in these polycyclic hydrocarbons. We find that a suitable arrangement of molecular fragments leads to this unusual behavior. This study shows a novel design strategy of polycyclic hydrocarbons with a large polyradical character, compelling synthetic targets for magnetic and spintronics materials.

Singlet fission results in the creation of a correlated pair of triplet excitons, making it an attractive phenomenon for devices that perform quantum computing tasks. While ultrafast optical techniques have proven insightful in understanding singlet fission and related processes, there is a need to experimentally probe them within a device context. Here, processes occurring on longer timescales, such as intersystem crossing, can become relevant and overlap with signals of singlet fission.

We access the spin dynamics of triplet excitons in organic field-effect transistors (OFETs) using magnetic field-modulated photoconductivity (MPC) measurements. We chose to use tetracene as the active layer because singlet fission in it has an energetically uphill component, as well as to enable comparison with a vast body of spectroscopic literature. Tetracene was grown as single crystals by the physical vapor transport technique to limit the effects of defects and microstructure. Measuring MPC as a function of temperature allows us to modulate the singlet fission rate, and applying a simple model gives insight into changes observed. Over the range of 350-170 K, the MPC signal, measured from -200 mT-200 mT, takes an M-shape with peaks at 8-10 mT and shoulders at 41-50 mT. These features are thermally activated, showing an MPC of 0% or slightly above at room temperature and decreasing until about 220 K. Our model suggests that this change is attributed to the thermal activation of singlet fission in tetracene. At the end points (+/- 200 mT), the much stronger decrease in MPC (as much as 11% at 220 K, with the exact value depending on the orientation of the triplet dipole with respect to the magnetic field) may be an indication that intersystem crossing is becoming a dominant spin-dependent process. Below 220 K, the MPC starts increasing again, which we attribute to a subtle phase transition. This work indicates that intersystem crossing should be considered when understanding photophysical processes in singlet fission devices.

Rapid exciton (energy) transport is critical for the realization of efficient organic semiconductor devices that surpass current efficiency limitations. Solution -based fabrication techniques for crystalline molecular thin films with macroscopic ordering show potential for overcoming the limitations of the molecular Frenkel exciton hopping transport. Once static/ structural disorder is removed from these films, spectroscopy experiments reveal the superluminescent signature of exciton delocalization in an entire class of small molecules where the packing motifs favor stronger intermolecular coupling.¹
In small molecule crystals the formation of delocalized and coherent excitons is the result of a complex balance between the long-range Coulomb interactions, short range π- stacking coupling and electron phonon coupling. The delocalization takes place when the latter is of the same order of magnitude as the nearest neighbor intermolecular coupling. Small energy phonon modes assist in the formation of coherent states while high energy molecular vibrations (dynamic disorder) introduce decoherence and localize the exciton.² The delocalization will favor ballistic energy transport through the system.
Here we report on how uniaxial strain induces a more robust coherent delocalized exciton state. The molecular thin films under study are deposited from solutions of metal-free octabutoxy Phthalocyanine (H₂OBPc) using a capillary pen - writing technique that results in macroscopic crystalline grains sizes, with molecules stacked in a J-like configuration along the [110] crystal axis.³ It was shown theoretically and experimentally that exciton delocalization only occurs along this molecular stack making the films a 1D excitonic system.⁴
Temperature, strain-dependent luminescence, absorption and linear dichroism microscopy indicate applying uniaxial strain along the stacking axis has a similar effect lowering the temperature by 100K, in terms of the prevalence of the coherent exciton state. The coherence length is estimate from the ratio between the delocalized exciton photoluminescence intensity and its first phonon replica. Increasing strain to 4.2 % initially shortens the coherence length at low temperatures, however, further increase in the strain results in a surprising recovering of the initial value of 22 molecular units (approx. 10 nm). This can be explained by the trend observed in the ratio between the intermolecular coupling and the electron-phonon interaction, also estimated from the temperature dependence of photoluminescence and the associated radiative lifetime. The applied strain results in stronger intermolecular interactions that maintain the delocalization up to a temperature of 200K.⁵
Finally, the mechanism of delocalized exciton formation is confirmed through strain -dependent transient absorption measurements and confocal photoluminescence microscopy that reveal the excited Frenkel excitons delocalize in less than 500fs. The dynamic disorder induces the formation of a more localized exciton-polaron state 5ps later. This process suffers significant changes in the presence of strain.
¹Rawat et al, J. Phys. Chem. Lett. 2015, 6, 1834–1840
²Fornari et al, J. Phys. Chem. C 2016, 120, 7987–7996
³Pan et al, Nat. Commun. 2015, 6, 8201
⁴Liang et al, J. Phys. Chem. C 2021, 125, 27966–27974
⁵Liang et al, J. Phys. Chem. C 2022, 126, 8889–8896

Physical structure creates electronic structure. By precise positioning of atoms through molecular chemistry we can design electronic structures for applications in quantum information science. We use ability to design both coarse and fine electronic structure through synthetic chemistry to create materials for quantum information science. Results on creating molecular color centers will be described.

Organic luminescent radicals present a new and exciting platform for exploring molecular functionalities at the interface of their unique optical and spin properties.[1] Near-unity internal quantum efficiency for red emission was demonstrated for donor-acceptor structures based on the tris(2,4,6-trichlorophenyl)methyl (TTM) radical.[2] This opens these materials towards applications in technologies ranging from organic light-emitting diodes and photovoltaics with eliminated triplet losses, to photomagnetic devices and molecular-scale information transfer.

Here we present a new family of molecules based on the TTM radical motif which contain high-multiplicity states in their emission mechanism. By combining light-induced EPR with ultrafast transient absorption and temperature-dependent transient luminescence spectroscopy we find a mechanism that uniquely supports both a high emission yield and a high yield of generation of the high-spin species. This is enabled by very low activation barriers between excited states, which arises from energy level matching in our molecular design. Pulsed EPR spectroscopy allows us to characterize the potential of our system for future quantum information science applications.

References
[1] Teki, Y., Chem. Eur. J. 26, 980 (2020)
[2] Ai, X., Evans, E.W., Dong, S. et al., Nature 563, 536–540 (2018)

Quantum information science (QIS) remains an important field of research due to its far-reaching implications in computing, communication and advanced sensing applications. Molecular systems are the most recent candidates for QIS, with several recent perspectives and reviews discussing the valuable properties unique to such systems. Coordination compounds provide precise tailoring of specific magnetic properties through structural tuning, and phthalocyanines have been established as a versatile system. Vanadyl phthalocyanine (VOPc) is considered here, a spin ½ system with long (μs) coherence times. It is easily and predictably deposited on a range of substrates and so has been directly implemented in resonant cavities to promote spin-photon coupling, placed on functionalized surfaces, or incorporated into hybrid architectures for sensing applications.

Read-out of the stored quantum information is a key requirement of all spin qubits and much work is being done on molecular compounds to achieve optical readout. Here, we investigate the possibility of a digital state readout by combining a molecular spin system to a valleytronic material. Films consisting of a few layers of transition metal dichalcogenides (TMDC) have demonstrated circular polarized photoluminescence (PL) from the transition to a direct band gap semiconductor. Chiral emission is relevant to spin-selective optoelectronics and combining the molecular spin qubit with a TMDC substrate could lead to interesting opportunities in QIS. However, at this stage little is known of the electronic and spin interactions at such an interface and the extent of spin-valley cooperativity. We therefore present a comparative magneto-optical study of the VOPc thermally evaporated on MoS₂, MoSe₂ and WSe₂ as a function of polarization. This provides critical information to design new architectures for quantum devices.

Quantum technologies benefit from atom-scale control of both the quantum bit and its local environment. In semiconductor materials, defects and dopants house localized electrons where the spin degree of freedom can be initialized and coherently controlled with a combination of optical and microwave signals. These semiconductor spin-qubits can be embedded in devices where their optical and spin properties can be enhanced by controlling both their electronic and nuclear-spin environment. Organometallic molecules provide an analogous platform for localized, coherent quantum states where both the qubit and its environment can be further modified via chemical synthesis. We have demonstrated optical addressability and coherent microwave control of the electron-spin ground-state of organometallic molecules containing a central chromium ion [1]. We have shown that the spin-optical interface of these molecular qubits can be tuned by modifying the structure and symmetry of the ligand field [1,2]. Atomistic modification of not only the qubit itself, but also its host environment can further be used to protect the qubit from magnetic field noise, for example due to the fluctuating electron or nuclear spin bath, yielding a five-fold improvement in spin coherence times by inducing noise-insensitive clock transitions [3]. This scalable approach to engineering quantum systems from the bottom up opens new avenues for tailoring synthetic qubits toward device-scale integration.

[1] S.L. Bayliss*, D.W. Laorenza* et al, Science 370, 1309 (2020)
[2] D. W. Laorenza et al, JACS 143, 50 (2021)
[3] S.L. Bayliss*, P. Deb*, D.W. Laorenza* et al, PRX 12, 031028 (2022)

Laboratory manipulation of single quantum emitters and single photons has matured to a routine procedure over the past two decades. Nevertheless, each solid-state emitter system (quantum dots, color centers, ions, etc) is still confronted with a range of material constraints that pose challenges for scaling and reliable use in devices. Our group has explored organic molecules in the class of polycyclic aromatic hydrocarbons (PAH) for about 25 years. In this presentation, I discuss our efforts of the last decade in coupling molecules to high-finesse Fabry-Perot open cavities, plasmonic nano-antennas and chip-based photonic circuits. We demonstrate dipole-induced transparency, strong coupling, single-photon nonlinearities as well as the cooperativity of two molecules via a common mode of a micro-resonator. Moreover, I present data on high-resolution spectroscopy of the vibronic transitions in single molecules as well as a theoretical proposal for a hybrid optomechanical platform that can lead to long coherence and storage times.

Molecular electronics offer several important advantages for spintronic, quantum sensing, and quantum computing fields. The ease of synthesis and fabrication combined with tunable spin physics makes some of these organic systems promising initial candidates for use in quantum information, however a key challenge is the ability to address individual molecular spins. Promising gate operations and coherence times have already been demonstrated with triplet exciton spin transfer using optical techniques, but electrical readout of these states would move these systems closer towards functional microelectronic circuits. Here we present a pathway towards electrical initialization and readout of triplet spins using the phenomenon of organic magnetoresistance (OMAR) with polycrystalline pentacene.

Pentacene and many of its derivatives and dimers, including 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), are studied for their ability to undergo a process known as singlet fission. Under illumination, these molecules readily excite to the first excited singlet state and efficiently and rapidly fission into two first excited triplet states onto neighboring molecules. This phenomenon has been fairly well explored for use in photovoltaics, and because of this, the process is well known to generate triplets of relative polarized spin due to their initial entanglement upon formation. Interactions of these polarized triplet spins with charge carriers creates a mixed state that holds information of both triplet spins. This triplet-charge spin-spin interaction is electrically detectable and forms the basis for singlet fission molecules to be explored for quantum information science. Polycrystalline pentacene is therefore a great model system for manipulation of these spin-spin interactions due to its large carrier mobility and ability to generate a high density of long-lived triplet excitations.

As the detection mechanism of these spin states, we look to theory of OMAR, where it has been shown that the interaction between triplets and mobile charges fundamentally changes the resistance through a thin film organic layer based on the applied magnetic field. We have fabricated polycrystalline pentacene devices that minimize the other commonly observed OMAR effects to isolate the response seen from these triplet-polaron encounters. Development of a quantum kinetic model through the application of the Stochastic Liouville equation of the triplet-charge mixed states reveals predictions that agree well with experiment. The model also provides predictions of the magnetoresistance when spin selective contacts are introduced, which would allow only anti-aligned triplet spins to quench, eventually purifying the triplet spins aligned with the applied field.

This triplet spin purification acts as an initialization process and can be achieved through a secondary thin film using a phenomenon known as chirality induced spin selectivity (CISS). This relatively newly discovered effect uses spiral-like chiral constituents to polarize either light or polaron spin of a specific orientation under an applied magnetic field. We show the effect of using the (S) and (R) chiral versions of (PEA)₂PbI₄ perovskite as a polarizing hole injection layer into polycrystalline pentacene. These results combined with detection via magnetoresistance provide promising first steps in the room temperature initialization and readout of molecular triplet spins.

Light can be confined through plasmonics to the nanoscale using coinage metals, particularly tightly within nm-wide dielectric gaps between metal facets. I will present recent advances in the nanoparticle-on-mirror concept [1] which creates robust and atomic-scale precision nanocavities that finally open up systematic exploration of many light-matter coupled phenomena. I will demonstrate how the size and shape of the nanoparticle facets play a key role, and can be controlled to modify in-/out-coupling of light. Enhanced structures can show hundred-fold increases in SERS emission (exceeding a million counts/mW/s) which enables high speed real-time interrogation of the dynamics of individual atoms and molecules in the nanocavities.
I will show a variety of phenomena from single de/protonation events [2], single molecule catalytic chemistry, and electrochromic functionalities [3], to the ability to create low-cost uncooled detectors of mid-IR light through upconversion in such plasmonic nanocavities [4]. I will also show that these structures can be simply wired up for a variety of molecular electronics and photoconductive devices [5,6]. I will introduce how optical forces are hundred-fold enhanced in such nanocavities [7-9]. I will finally discuss the prospects for such nanoscale building blocks across a variety of fields.

[1] Extreme nanophotonics from ultrathin metallic gaps, Nature Materials 18, 668 (2019); DOI: 10.1038/s41563-019-0290-y
[2] Tracking interfacial single-molecule pH and binding dynamics..., Science Advances 7:eabg1790 (2021); DOI: 10.1126/sciadv.abg1790
[3] Scalable electrochromic nano-pixels using plasmonics, Science Advances (2019); DOI: 10.1126/sciadv.aaw2205
[4] Detecting mid-infrared light by molecular frequency upconversion in dual-wavelength nanoantennas, Science (2021); DOI: 10.1126/science.abk2593
[5] Optical probes of molecules as nano-mechanical switches, Nature Comm 11:5905 (2020); DOI: 10.1038/s41467-020-19703-y
[6] Quantum Tunneling Induced Optical Rectification and Plasmon-Enhanced Photocurrent in Nanocavity Molecular Junctions, ACS Nano (2021); DOI: 10.1021/acsnano.1c04100
[7] Optical suppression of energy barriers in single molecule-metal binding, Science Advances 8: eabp9285 (2022); DOI: 10.1126/sciadv.abp9285
[8] Picocavities: a primer, Nano Letters 22, 5859 (2022); DOI: 10.1021/acs.nanolett.2c01695
[9] Single-molecule optomechanics in picocavities, Science 354, 726 (2016); DOI: 10.1126/science.aah5243

Diverse and unique functionalities of molecules have extended their applications beyond electronics to fields including optics, plasmonics, mechanics, spintronics, and quantum technologies. Molecular devices with structurally-tunable, nanometer-thin critical dimensions are highly desirable for their extended sensitivity, energy efficiency and speed. Building such molecular platforms reliably, however, has been a challenge due to the fabrication limitations posed by top-down techniques.

Here, we present nanoparticle contact printing as a technique for scalable fabrication of molecular junctions in the form of mechanically active plasmonic resonators where the molecules serve as nanoscale reconfigurable springs with sub-nm control. The plasmonic resonator is configured as a nanoparticle-on-mirror structure, where a compressible dielectric molecular spacer is sandwiched between a metallic nanoparticle and a metallic film. To achieve scalable patterning of these active resonators, we developed nanoparticle contact printing, where >2000 colloidal plasmonic gold nanoparticles are deterministically patterned onto template-stripped gold with sub-50 nm control over positional accuracy. The resulting nanoparticle-on-mirror constructs are highly sensitive to sub-nm structural changes in the molecular junction. We demonstrate the dynamic mechanical tunability of the molecular junction and its limits by testing the resonators under thermal cycling, where the molecular layer undergoes thermal expansion and recovery. The ~1.2 nm spectral tunability observed upon heating reflects a mere ~30 pm change in the molecular gap, highlighting the sensitivity and precision of this platform. Such tunable structures, whose response can be modified through choice of the molecular layer, can form building blocks for mechanically active molecular devices, enabling high-throughput statistical and correlative characterization of the molecular mechanics, and integration into emerging nanoscale sensors, actuators, reconfigurable optical devices, and tunable metasurfaces.

In recent years incredible strides have been made in the development of molecular electronic systems that possess unique functionality. By combining chemical design with physical modeling and electrical characterization techniques it has become clear that molecules are capable of a wide range of impressive electronic functions that extend far beyond the development of standard devices such as transistors and diodes. An array of electromechanical, electrochemical, thermoelectric, and quantum devices now provide promise for memory devices, sensors, and multi-state logic units which could yield new paradigms for in-memory computing, various post von Neumann architectures, or for chemical and biological sensing systems. But, despite these possibilities, one of the major issues that arose in the nascent days of molecular electronics still lingers and limits its ultimate utility. That issue is integration. Despite a wide range of unique devices, and novel chemical and physical properties, it has remained difficult to integrate these materials into a larger-scale system in a way that is reliable, reproducible, and eventually manufacturable. In this talk we will discuss emerging approaches aimed at moving molecular-scale electronic systems from the lab and into applications. To integrate top-down lithographic approaches with bottom-up self-assembly methods we utilize novel micro-electromechanical systems (MEMS) that allow robust single-molecule electrical measurements at the chip-level; the programmability imparted by DNA nanotechnology to create novel nanoscale electrical and lithographic systems; and finally the utility of carbon nanotubes for making secure and robust contact to a single-molecule to allow facile integration with traditional photolithographic processes.

Reliable molecular junctions require high-quality self-assembled molecular monolayers. To form such monolayers, clean, ultra-smooth (< 1 nm root-mean-square roughness) growth surfaces are required. One approach to such surfaces is template stripping, where the material of interest is deposited on a template, such as a silicon wafer, then delaminated from the underlying substrate, revealing a clean, ultra-smooth surface suitable for molecular assembly. Conventionally, this template stripping approach has been limited to large-area surfaces of metals such as Au, Ag, Pd and Pt, which naturally have low adhesion to the underlying silicon surface, allowing for delamination. Though suitable for fundamental studies, these large-area surfaces of low-adhesion noble metals have limited prospects for integration into molecular devices and systems.

Here, we demonstrate template stripping of diverse materials (Au, Pt, Al, SiO₂, Al₂O₃) including and beyond those achievable through conventional template stripping approaches. We overcome the requirement for a low-adhesion material by functionalizing the underlying silicon substrate with molecular release layers that enable reliable stripping regardless of material adhesion, while maintaining the surfaces ultrasmooth (< 1 nm). This expands the use of template stripped surfaces to the assembly of diverse molecular monolayers beyond those readily compatible with assembly on conventional, noble metal surfaces. This approach further opens the possibility of integrating molecules with materials, beyond noble metals, that display functionalities, such as spintronic or semiconducting, critical for the emerging molecular devices. We further demonstrate lithographic patterning of template stripped surfaces to realize ultra-flat, ultra-smooth surfaces composed of multiple materials. These planarized heterogenous surfaces allow for orthogonal assembly chemistries to be run on the same substrate, opening possibilities for fabrication of more complicated molecular device architectures. Because our template stripping approach mirrors conventional wafer bonding, we discuss its prospects for scalable fabrication of substrates suitable for molecular electronic devices on an industry-compatible toolset.

Graphene is believed to be an optimal material for a promising new generation of nanoscale devices using spin as information carrier, storage and processing [1]. During the last years, atomically precise synthesis of poly-aromatic hydrocarbons (PAH) has been developed [2-3] allowing new possibilities to engineer their transport properties and controll the separations between the spins. The intrinsic pi-paramagnetism emerging spontaneously as radical states in open-shell graphene structures give rise to a more delocalized, mobile and isotropic magnetism than electrons in d-orbitals. Nonetheless, graphene-based nanomaterials also emerge as an ideal solution to combine excellent spin transport properties with large diffusion length and long coherence time needed for quantum spintronic applications with energy scales compatible with room-temperature operations.
We report on manipulation of spin-spin interactions for PAH di-radical molecules [4] in a mechanically controlled break junction (MCBJ) and in 3-terminal device (electro-migration break junction EMBJ) at low temperature. Magnetic fingerprints of the molecules manifest themselves by zero-bias peaks in the differential conductance spectra attributed to a Kondo resonance arising from hybridization of one of the radical electrons with the metallic electrodes and spin-flip inelastic electron tunnelling spectroscopy (IETS) steps originating from the singlet-triplet gap of the free electrons [5]. Varying the distance between the electrodes in the MCBJ affects the coupling strength and varying the gate voltage in the EMBJ affects the energy level alignment with the Fermi level of the electrodes. Thus, by mechanical manipulation and electrostatic gating the spin signature of the molecule can be adjusted and controlled. This study provides new insight on the interplay between magnetic fingerprints of all-organic molecules embedded in solid-state devices.

References:
[1] Hirohata, A., Hillebrands B. et al., J. Magn. Magn. Mater. 166711 (2020).
[2] Pavliček, N., Mistry, A., Majzik, Z. et al., Nature Nanotech 12, 308–311 (2017).
[3] Zhang, X., Xu, Z., Si, W. et al., Nat Commun 8, 15073 (2017).
[4] Baum, T. Y., Fernández, S., Peña, D., van der Zant, H. S. J, Nano Lett. 22, 20, 8086–8092 (2022).
[5] Ternes, M., New J. Phys. 17 063016 (2015).

This project received funding from the European Union Horizon 2020 research and innovation program under grant agreement N°863098 – SPRING. The molecules used were synthesized by the group of Diego Peña from Universidade de Santiago de Compostella.

Graphene nanoribbons (GNRs) synthesized from the bottom up with molecular precursors have enabled the capability of engineering the electronic, optical, magnetic and quantum physical properties with atomics scale precision, which promises novel functionalities and next-generation computing devices. This talk presents recent progress and remaining challenges of the technology integration of bottom-up synthesized GNRs into field-effect transistors (FETs). Several key technology components for GNR FETs are covered, include band-gap engineering, synthesis and alignment of long-length GNRs, achieving low-resistance electric contacts, and high-k gate stack integration. A statistical approach that combines theoretical modeling and experimental characterization is developed to evaluate and benchmark the device performance. Remaining challenges and future research directions are discussed for the further improvement of device performance of GNR FETs.

Since the start of the 2019 pandemic (COVID-19), the development of rapid and cost-effective testing has been an ongoing effort to help combat this highly transmittable and infectious disease. Over the past two years, the virus has continued to mutate and evolve, and several variants of SARS-CoV-2 have emerged which have reduced the vaccines’ effectiveness. Therefore, there is a significant need for surveillance of the SARS-CoV-2 variants of concern to track the spread of the disease and help to confine the virus. Currently, the standard identification method for the subvariants utilizes reverse-transcription polymerase chain reaction (RT-PCR). However, this method is not suitable for rapid, high throughput testing of large populations, it is costly, time-consuming, and requires expert staff. Here we present an alternative to RT-PCR based on single-molecule conductance measurements which open new avenues for sensitive and cost-effective detection of the SARS-CoV-2 variants of concern.
Previous studies using the single-molecule break junction (SMBJ) approach have demonstrated that this technique is capable of detecting and identifying RNA: DNA hybrids via their conductance values. This work focuses on identifying SARS-CoV-2 variants of concern based on the codon sequences that represent mutations in the spike protein. Our hypothesis is that using a specific DNA probe that is complementary to the RNA for every mutation would result in a specific conductance value that represents the presence of the variant in the sample. Therefore, we can detect and identify each variant in a mixed sample based on their unique electronic fingerprints.
We focus on the alpha, beta, and delta variants which according to the World Health Organization (WHO) are classified as SARS-CoV-2 variants of concern. We design a series of 12 base-pair DNA probes that contain the mutation and would bind to the corresponding RNA section of the alpha and beta variant genome block. Our experimental results indicate that in the presence of the mutation, the conductance value of the hybrid RNA: DNA sequence is significantly different from when there is no mutation present. We further utilize the XGBoost machine learning classifier to aid in the rapid detection of the SARS-CoV-2 variants. This work demonstrates the utility of the SMBJ technique for the accurate detection and identification of SARS-CoV-2 variants with the potential for real-time application.

Throughout history, man has exploited matter to carry out tasks well beyond his biological constraints. Starting from primitive tools with functionality solely derived from shape and structure, we have moved on to responsive matter that can change its properties upon external stimulus and even further to adaptive matter that can change its response depending on the environment. One of the grand scientific and intellectual challenges is to make matter that can actually learn. Such matter’s behavior would not only depend on the here and now, but also on its past. It would have memory, autonomously interact with its environment and self-regulate its action. We may call such matter ‘intelligent’.
Here we introduce the concept of “intelligent matter”¹ and discuss a number of experiments on disordered nanomaterial systems, where we make sure of “material learning” to realize functionality. We have shown that a ‘designless’ network of gold nanoparticles can be configured into Boolean logic gates using artificial evolution². We further demonstrated that this principle is generic and can be transferred to other material systems. By exploiting the nonlinearity of a nanoscale network of boron dopants in silicon, referred to as a dopant network processing unit (DNPU), we can significantly facilitate classification. Using a convolutional neural network approach, it becomes possible to use our device for handwritten digit recognition³. An alternative material-learning approach is followed by first mapping our DNPU on a deep-neural-network model, which allows for applying standard machine-learning techniques in finding functionality⁴. We also show that the widely applied machine-learning technique of gradient descent can be directly applied in materia, opening up the pathway for autonomously learning hardware systems⁵. Finally, we show that kinetic Monte Carlo simulations of electron transport in DNPUs can be used to reproduce the main characteristics and to depict the charge trajectories⁶.

[1] C. Kaspar et al., Nature 594, 345 (2021)
[2] S.K. Bose, C.P. Lawrence et al., Nature Nanotechnol. 10, 1048 (2015)
[3] T. Chen et al., Nature 577, 341 (2020)
[4] H.-C. Ruiz Euler et al., Nature Nanotechnol. 15, 992 (2020)
[5] M.N. Boon et al., arxiv.org/abs/2105.11233 (2021)
[6] H. Tertilt, J. Bakker, M. Becker, B. de Wilde et al., Phys. Rev. Appl. 17, 064025 (2022)

This talk will exemplify our on-going interest and great effort in developing efficient lithographic methodologies capable of creating molecular electronic devices through the combination of top-down micro/nanofabrication with bottom-up molecular assembly. These devices use nanogapped nanomaterials (such as silicon nanowire (SiNW) and graphene) as leads or probes formed by electron beam lithography and precise oxygen plasma etching. Through robust chemical linkages, functional molecular bridges terminated with active moieties are covalently wired into the functionalized graphene/SiNW nanogaps to form stable single-molecule junctions with desired functionalities. We have used these approaches to reveal the dependence of the charge transport of individual metallo-DNA duplexes on p-stacking integrity, and fabricate molecular devices capable of realizing label-free, real-time electrical detection of biological interactions at the single-event level, or switching their molecular conductance upon exposure to external stimuli, such as ion, pH and light.
References
[1] C. Jia, X. Guo, et al., Science, 352,1443 (2016).
[2] D. Xiang, X, Wang, C. Jia, T. Lee, X. Guo, Chem. Rev., 116, 4318 (2016).
[3] N. Xin, J. F. Stoddart, X. Guo, Nat. Rev. Phys., 1, 211 (2019).
[4] Y. Li, C. Yang, X. Guo, Acc. Chem. Res., 53, 159 (2020).
[5] C. Jia, B. Ma, N. Xin, X. Guo, Acc. Chem. Res., 48, 2565 (2015).
[6] C. Jia, X. Guo, Chem. Soc. Rev., 42, 5642 (2013).
[7] A. Feldmen, M. L. Steigerwald, X. Guo, C. Nuckolls, Acc. Chem. Res., 41, 1731 (2008).
[8] D. Su, X. Guo, et al., Adv. Sci., 2200022 (2022).
[9] X. Guo, C. Nuckolls, et al., Nat. Nanotechnol., 13, 163 (2008).
[10] J. Wang, X. Guo, et al., Angew. Chem. Int. Ed., 53, 5038 (2014).

In our work, we demonstrate that tetrathiafulvalene (TTF), which is an important building unit for functional materials, can act as a new anchor for molecular devices. We show that, in TTF-fused naphthalene diimide molecular wires, the sulfur atoms in TTF directly bind to gold electrodes and form single Au-molecule-Au junctions. The unique TTF-Au contacts created via Au-S bonding enable highly efficient hybridization of Au orbitals and molecular conducting π-channels, yielding strong electrode-molecule coupling and remarkably high conductance. Moreover, we introduce additional thiohexyl (SHe) anchors to the TTF unit and show that the multiple Au-S binding sites in these molecular wires offer mechanically switchable binding modes through either TTF-Au or SHe-Au interaction. This leads to mechanically tunable metal-molecule electronic coupling and conductivity in single molecule devices. These findings provide a new strategy for building highly tunable molecule-metal interfaces for molecular devices.

Carbon nanorings cycloparaphenylenes (CPPs) and carbon nanobelts (CNBs) are typical hoop-shaped nanocarbons that have unique radially oriented π-systems. This talk will summarize our recent work on incorporating these unique nanocarbons into molecular electronic devices using the scanning tunneling microscope-break junction technique. We show that, compared with traditional linear molecular devices, the hoop-shaped CPP and CNB based devices have significantly higher conductance. DFT-based calculations elucidate that the high performance of these devices arises from the distorted, radially π-conjugated molecular structures. These findings demonstrate the potential of using hoop-shaped conjugated nanocarbons to achieve unprecedented electronic performance, and may stimulate future design of nanoscale devices based on unconventional molecular geometries.

Electrophilic aromatic substitution (EAS), which involves the formation of charge-transfer complex between electrophiles and the electron-rich aromatic rings, is one of the most fundamental reactions in organic chemistry. Due to the relatively large energy barrier for the charge transfer activation, EAS reactions are often limited to systems containing highly activated (nucleophilic) nuclei or very strong Lewis acid catalysts. Here, instead of traditional chemical reagents, we demonstrate the utilization of an oriented external electric field (OEEF) to catalyze the EAS reactions based on the scanning tunneling microscope break junction technique. Using cycloparaphenylenes with unique radially π-conjugated structures as the aromatic substrates, we can apply the OEEF perpendicular to the aromatic ring, which is crucial for harnessing the field effect. Under a very mild condition (a voltage of <1 V), the applied OEEF can significantly accelerate the EAS by the otherwise unreactive Au electrophile, cleaving the inherently inert C(sp²)-C(sp²) bond and leading to high-yield (~97%) formation of linear oligophenylenes terminated with covalent Au-C bonds. These results not only demonstrate the feasibility of OEEF catalysis of EAS, but also offer a way of exploring new mechanistic principles of classic organic reactions aided by OEEF.

Controlling heat flow is an important goal for materials, for example, to thermally insulate components or, conversely, to allow heat to rapidly dissipate. One area where this is technologically relevant is the microelectronics industry where computer chips are limited both by the challenges of miniaturization but also thermal management. At the same time, chemical control of heat transport is not part of traditional chemistry, where there has been much more focus on the electronic processes involved in making and breaking chemical bonds, so we cannot rely on established chemical knowledge to design new thermal management materials. Today, we need new methods and understanding to broaden our chemical intuition for thermal transport properties of molecules.
In this talk I will outline our efforts to develop chemical intuition for how we can control heat transport in molecules. While our understanding is far from complete, we can outline some general principles. Structural disorder in flexible systems, perhaps unsurprisingly, results in vibrational mode localization and reduced heat transport. While one might expect that breaking chemical bonds to yield non-bonded, self-assembled systems, would reduce heat transport it can also increase heat transport in certain cases. Finally, mass-disorder, for example from by changing to an inorganic complex instead of an organic molecule can be very effective at suppressing heat transport. Less clear are the effects of changing the structure of an organic molecule, for example between conjugated and saturated systems, leaving open a broad area of uncertainty as to ideal systems.

The interaction between magnetic impurities and superconductors leads to fascinating physical phenomena resulting from the competition between Kondo screening and Cooper pair formation [1]. To this end, individual magnetic impurities can form states within the superconducting gap, called Yu-Shiba-Rusinov (YSR) states [1,2]. YSR states are of great interest because they have the potential to realise topological superconductivity. Here we show that such YSR states form in a neutral and stable all-organic radical molecule coupled to proximity induced superconducting break-junction electrodes. We experimentally study the thermoelectric response [3] of the system at mK temperatures, both in the YSR regime and – by applying magnetic fields – in the Kondo regime [4]. Ultimately, we observe a two-fold increase of the thermoelectric efficiency which is induced by the YSR states. This study highlights the power of thermocurrent measurements as a new spectroscopic tool to study nanoscale devices, and reveals new strategies for engineering highly efficient thermoelectric energy conversion at cryogenic temperatures.

[1]K. J. Franke et al., Science 332, 940-944 (2011)
[2]J. O. Island et al., Phys. Rev. Lett. 118, 117001 (2017)
[3]P. Gehring et al., Nat. Nanotechnol. 16, 426-430 (2021)
[4]C. Hsu et al., Phys. Rev. Lett. 128, 147701 (2022)

Thanks to their sub-10 nanometre size, molecules offer a fantastic platform for exploiting quantum effects at room temperature. Recently, it was shown that molecular wires can mediate long-range, phase-coherent tunnelling with remarkably low attenuation beyond a few nanometres, even at room temperature [1]. This opens up the possibility of using quantum interference (QI) to control the transport of electrons, spins and phonons through molecular junctions consisting of a single or a few molecules between electrodes.

In this talk I will present evidence for room-temperature QI in molecular junctions and discuss the agreement and disagreement between experiments and predictions based on phase-coherent quantum transport theory [2]. Then, I will discuss strategies to control QI in molecular junctions [3] including chemical and structural modifications of molecules, and the application of electric field or light. I will then discuss how these strategies can be used to improve the efficiency of thermoelectric materials for energy harvesting and cooling, to improve spin filtering through molecules for quantum information applications, and to selectively sense biological species.

References
[1] Chem. Mater. 25 (21), 4340, 2013; Nano Lett. 22 (18), 7682, 2022.
[2] PNAS, 112(9), 2658, 2015; Nature Materials, 18(4), 364, 2019; JACS, 137(13), 4469, 2015; Nano Lett., 18(7), 4482, 2018; JACS, 140(40), 12877, 2018; Nanotechnology, 29, 373001, 2018.
[3] Nano Lett., 22, 3, 948, 2022; JPCL, 13(39), 9156, 2022; Nanoscale Adv., 2(3), 1031, 2020; Nature Comm. 11, 5905, 2020; Angewandte Chem, 61, e2021169, 2022; ACS Nano, 14(5), 5754, 2020; Nanomaterials 10 (8), 1544, 2020; JACS, 143(25), 9385, 2021; PCCP, 21, 2378, 2019; Nano Lett., 20(11), 7980, 2020.

Artificial intelligence (AI) has been heralded as the flagbearer of the fourth industrial revolution but comes with a cost and that is computing power. It is projected that by 2040, we will need more computing energy than the total energy we can produce now. So, we need devices that can offer higher computing/ storage density with low energy consumption like neurons. We are addressing these challenges using oxide and molecular-electronic based memristors, which enable us to overcome the von Neuman bottleneck by co-locating the memory and computing functions on the same device, as in neuromorphic computing.
I will discuss a variety of strategies for forming oxide based memristors using different phenomena- band filling and creating a quasi-conduction band, using oxygen vacancies to create conductive percolation, using metal-insulator transitions, or using asymmetric tunneling at a ferroelectric barrier. The relative merits of the various approaches will be detailed.
In addition, I will touch upon memristive devices and circuits made from an azo-aromatic complex with extra-ordinary reproducibility, robustness, and scalability. These devices have been shown to switch with energies approaching atto-joules with measured switching times shorter than 5 ns. By using a simple cross bar array, we have shown that these devices perform with an energy-speed product 5000 times that of a state-of-the-art CMOS circuit.

References
1. Sreetosh Goswami*, Rajib Pramanick, Abhijeet Patra, Santi Prasad Rath, Ariando, Damien Thomson, T. Venkatesan*, Sreebrata Goswami* and R Stanley Williams*, Decision Trees within a Molecular Memristor, Nature, Sep. 3, 2021
2. Sreetosh Goswami, Adam J. Matula, Santi P. Rath, Svante Hedstrom, Surajit Saha Meenakshi Annamalai, Debabrata Sengupta, Abhijeet Patra, Siddhartha Ghosh, Hariom Jani, Soumya Sarkar, Mallikarjuna Rao Motapothula, Christian A. Nijhuis^, Jens Martin, Sreebrata Goswami*, Victor S. Batista*, T. Venkatesan*, Robust resistive memory devices using solution-processable metal-coordinated azo-aromatics, Nature Materials, 16, 1216 (2017). News and views on this article: I. Valev and M. Kozicki, Non-volatile Memories: Organic Memristors come of age, Nature Materials, 16, 1170 (2017)
3. Rui Guo, Weinan Lin, Xiaobing Yan, T Venkatesan, Jingsheng Chen,
Ferroic tunnel junctions and their application in neuromorphic networks, Applied Physics Reviews, 2020/3/6
4. C. J. Li, L. Huang, T. Li, W. M. Lü, X. P. Qiu, Z. Huang, Z. Q. Liu, S. W. Zeng, R. Guo, Y. L. Zhao, K. Zeng, J.MD. Coey, J.S. Chen, Ariando, T. Venkatesan* Ultrathin BaTiO₃-based Ferroelectric Tunnel Junctions through Interface Engineering. Nano Letters 15 (4) 2568–2573 (2015)