The formation of binary superlattices via colloidal crystallization is the most promising method to realize nanostructured solids in which different materials are in near contact and ordered in a well-defined 3-D geometry. Despite the obvious importance for novel applications in opto-electronic materials, studies of nanocrystal superlattices consisting of two types of semiconductor quantum dots have been quite limited (1-3). In this presentation we report on the self-organization of PbSe and CdSe nanocrystals into binary superlattices upon evaporation of the solvent from a mixed suspension at elevated temperatures and reduced pressure (3). We show that, besides the choice of the solvent, the size-and concentration ratio of both types of nanocrystals are the relevant parameters determining the composition and structure of the superlattice. We demonstrate AB, AB2, AB5 and AB13 structures (A=PbSe, B= CdSe) obtained by varying the diameter ratio PbSe/CdSe between 0.5 and 0.8. We show that binary superlattices can have an intricate structure that is best resolved by 3-D TEM (tomography). The results are discussed on the basis of newly developed simulation models based on hard-sphere building blocks. The electronic properties of a number of PbSe/CdSe NC superlattices are currently under study. 1) J. J. Urban et al., Nature Mat. 6, 115 (2007)2) Z. Y. Chen et al. J. Am. Chem. Soc. 129, 15702 (2007)3) K. Overgaag et al., J. Am. Chem. Soc. 130, 7833 (2008)

Hybrid materials composed of conjugated polymers and semiconductor nanocrystals are promising systems for the fabrication of low cost, large area photovoltaic devices based on the so-called bulk-heterojunction concept. However, in simple blends of both components, generally undesired phase segregation on a submicron level occurs. We present a new approach to overcome this problem and to achieve morphology control of hybrid material thin films. It is based on the molecular recognition process via hydrogen bonding between complementary chemical groups in the side chain of the polymer and on the nanocrystal’s surface.A new copolymer was synthesized, namely poly(3-hexylthiophene) bearing oxydiaminopyrimidine side groups, which are capable of forming triple hydrogen bonds with mercaptohexylthymine-capped CdSe nanocrystals. Due to their different solubility parameters, the functionalized components can be directly deposited on various substrates (glass, ITO, silicon oxide) using the layer-by-layer method. This technique, based on the alternated dipping of the substrate into the polymer solution and into the NCs’ dispersion, allowed us to control the composition and thickness (range: 30-150 nm) of the hybrid films on a molecular level. In addition, the materials’ consumption is significantly reduced in the dip-coating process with respect to the deposition via spin-coating, usually applied for the processing of hybrid materials.Complementary SEM and SAXS investigations of the deposited layers indicate that the polymer and the nanocrystal phases form a quasi-interpenetrating network, which is favourable for their application in thin film solar cells relying on the bulk heterojunction concept. First device tests clearly show the photovoltaic effect with conversion efficiencies in the typical range (0.1% for an active surface of 28 mm2 under AM 1.5 conditions using 100 mW/cm2 simulated white light) observed for blends of 5 nm spherical CdSe NCs with poly(hexylthiophene) or MEH-PPV. Future device improvement is expected through the optimization of the number of deposited bilayers, NCs’ composition, shape and surface chemistry, to list the principal parameters.

The “bottom-up” methodology consists of designing recognition into molecules that then self-assemble into the desired nano-architecture in the appropriate microenvironment. Peptides undergo self-assembly in solution to form hierarchal nanostructures. Herein, we demonstrate the use of a peptidic template for the construction of parallel, linear arrays of inorganic nanoparticles. A 20 amino acid peptide, consisting of alternating hydrophilic (lysine) and hydrophobic (valine) residues flanking a central diproline turn sequence (VKVKVKVKVPPTKVKVKVKV-NH2) was employed as a nano-scale template for the organization of 2nm gold particles. This peptide self assembles into laminated fibrillar morphology in solution and has a periodic nanostructure consisting of alternating hydrophobic and hydrophilic layers with a lateral periodicity of 2.5 nm. Negatively charged gold nanoparticles are templated into the positively charged lysine layer through electrostatic interaction and are aligned within the template that itself swells to a periodic spacing of 4.0 nm in order to accommodate the particles. These 1D nanoparticle arrays have potential applications in fields like nano-electronics, and we are currently attempting to create arrays of quantum dots and hetero-structures of metal and semiconductor particles.

Nanoparticles are indispensable ingredients of solution-based optical, dielectric, and catalytic thin films. While solution-based methods are promising low-cost alternatives to vacuum methods, they can have significant limitations. Coating uniformity, thickness control, roughness control, mechanical durability, and incorporation of a diverse set of functional organic molecules into nanoparticle thin films are major challenges.We have used the electrostatic layer-by-layer (LbL) assembly technique to make uniform, conformal multi-stack nanoparticle thin films for optical applications with precise thickness control over each stack. Two particularly sought-after optical applications are broadband antireflection (AR) and structural color. The effects of inter-stack and surface roughness on optical properties of these constructs (e.g., haze and spectral response) have been studied quantitatively using a combination of Fourier-transform methods and atomic force microscopy (AFM) measurements. Deconvoluting root-mean-square (RMS) roughness into its large-, intermediate-, and small-scale components enables enhanced optical simulations. A 4-stack broadband AR coating (<0.5% average reflectance in the visible range, and 0.2% haze) composed of alternating high-index (n~2.1) and low-index (n~1.3) stacks has been made on glass substrate. Films calcinated at 550°C endure a one-hour-long cloth cleaning test under 100 kPa normal stress.Beyond the direct assembly of multi-stack nanoparticle thin films, it would be highly desirable to identify post-assembly modification methods that supplement thickness and roughness control with chemical functionalization. Hydrophilic, hydrophobic, reactive, or inert chemical vapors capillary-condense in the vicinity of contact points between nanoparticles with equal facility. This phenomenon can be used advantageously to functionalize nanoparticle assemblies. The volume fraction of the condensate is particle size-dependent, which allows targeted functionalization in a multi-stack LbL assembly. 2-stack films composed of 8nm and 50nm SiO₂ particles on the bottom and top stacks, respectively, were functionalized with oligomers (e.g., PDMS) or UV-sensitive monomers (e.g., tri(ethyleneglycol) dimethacrylate (TEGDMA)). The condensate volume fraction in the bottom stack was significantly higher than that in the top stack, and excellent broadband AR functionality (<0.6% average reflectance in the visible range) resulted from a mere 160nm-thick film. UV-crosslinking ability was demonstrated for TEGDMA-functionalized films. The PDMS-functionalized film was also assembled on epoxy microlenses, and showed no optical or mechanical failure upon repeated rapid (30s) heating-cooling cycles between 25°C and 260°C. Finally, we show that capillary condensation can be used to enhance mechanical durabilities of otherwise delicate nanoparticle assemblies, as well as to prevent their physical-chemical vulnerability to moisture.

Significant progress in the synthesis of narrow-gap quantum dots, such as PbTe nanocrystals, has triggered recognition of their potential in photovoltaic, thermovoltaic, and thermoelectric applications. Some of the recent advances in enhancing the thermoelectric figure of merit are associated with these materials, due to the strong quantum-confinement effect as well as large phonon scattering. It is essential to develop techniques to fabricate nanostructures containing quantum dots to obtain not only the desired internal microstructures and grain boundary properties, but also to simultaneously maintain the nanoscale particle size. Furthermore for building devices, the site-specificity of the technique and its compatibility with traditional microfabrication procedures are also required. In this study, a versatile nanopatterning approach, soft-eBL, is being employed to prepare various nanostructures. By collectively combining electron beam lithography and deposition of solution precursors of narrow-gap quantum dots, nano-rings/dots/lines of PbTe or/and SnTe are fabricated. By tuning preparation conditions, the nanocrystals are directed to exhibit either short range or long range packing order, which has been confirmed by high-resolution scanning electron microscopy (HRSEM) and scanning transmission electron microscopy (STEM). Quantum-dot nanolines, different from thin-film superlattices, introduce the confinement in one more dimension, which have been confirmed by theoretical study to show higher figure of merit. Chemical treatment with hydrazine and mild heating are performed to increase the electrical conductivity while maintaining high phonon scattering, and thus to increase the thermoelectric figure of merit. Moreover, the nanocrystals with n- or p-typesemiconducting property are also being built into field-effect transistors, which serve as good alternatives to traditional semiconductor circuits. The presentation will cover aspects of soft-eBL nanopatterning and measurements of colloidal assembly of thermoelectric nanostructures.

Nanocrystal quantum dots (NQDs) are nanosized crystalline semiconductor particles that show near-unity photoluminescence quantum yields and size-dependent emission colors tunable through the quantum-confinement effect. Because of these properties, nanocrystals are attractive materials for various light-emitting applications including optical amplification and lasing. Due to the almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron-hole pairs (excitons), optical gain can only occur when nanocrystals contain at least two excitons. A complication associated with this multiexcitonic nature of light amplification is fast, picosecond optical-gain decay induced by nonradiative Auger recombination, in which one exciton recombines by transferring the energy to the other. A few years ago, it was demonstrated that close-packed films of NQDs can still show both optical gain and amplified spontaneous emission (ASE) when excited with short and intense laser pulses [1]. However, the ASE thresholds are quite high (several mJ/cm²), which seriously hinders lasing applications of NQDs. Here, we present new results from our studies of dense films made of a new type of nanocrystals dubbed “giant” quantum dots (g-NQDs) [2]. These NQDs comprise an emitting core particle of CdSe overcoated with a very thick shell (up to 20 monolayers) of wider-gap CdS. For this new type of NQD, we observe that the ASE threshold drops down to just a few μJ/cm², which is almost three orders of magnitude lower than that previously reported for CdSe NQDs. We explain this result by a significant increase in the absorption cross-section of g-NQDs compared to traditional nanocrystals and lengthening of biexciton lifetimes. We also observe other unusual optical-gain behaviors for these structures such as multi-band ASE spectra, in which the band-edge emission co-exist with stimulated emission features due to transitions involving excited electronic states. The overall spectral range of optical amplification extends over more than 500 meV; such broad-band ASE has never been previously observed for other types of optical gain media. These results demonstrate that g-NQDs are very promising materials for applications in practical lasing technologies.[1] V. I. Klimov, A. A. Mikhailovsky, Su Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H.-J. Eisler, M. G. Bawendi, Optical Gain and Stimulated Emission in Nanocrystal Quantum Dots, Science 290, 314 (2000).[2] Y. Chen, J. Vela, H. Htoon, J. L. Casson, D. J. Werder, D. A. Bussian, V. I. Klimov, J. A. Hollingsworth. “Giant” Multishell CdSe Nanocrystal Quantum Dots with Suppressed Blinking, J. Am. Chem. Soc. 130, 5026 (2008)

The emergence of colloidal quantum dot-based light emitting diodes (QD-LEDs) offers a great prospect for developing low-cost, efficient, bright, color-saturated, large-area color displays compatible with flexible substrates. An imminent hurdle along the roadmap to QD-LED-based displays is, however, the lack of an appropriate technology to efficiently deposit and pattern QD-layers with precise controls over thickness, composition, surface morphology, and resolution needed to fabricate RGB-pixel arrays of bright QD-LEDs over large surface areas for passive/active matrix displays. Compared to most solution-processed organic LEDs, the efficiency and brightness of QD-LEDs are highly dependent on the thickness of the emissive QD-layer in the device active region. To date, almost all the record-performances of QD-LEDs have been achieved with devices containing QDs of 1-5 monolayer (ML)-thickness. Thicker QD-active regions result in an increase of the operating voltage and decrease of the carrier-injection efficiency due to the slow dot-dot transport. In this conference we will introduce a process of mist-deposition which allows for the simultaneous assembly and patterning of QDs in QD-LED fabrication with precise controls over thickness. The technology of mist deposition was originally developed for the liquid-source chemical deposition of ultra-fine films of ferroelectrics, high-k dielectrics for MOS gates and other applications in the microelectronics industry. It was adapted in the present work to deliver emissive QDs for the formation of ultrathin active layers in QD-LEDs. During the deposition submicron mists of QD solutions are softly and uniformly transported onto the substrate surface, where they subsequently coalesce at a controlled rate, allowing for a higher degree of thickness- and morphology-control as well as lower waste as compared to spin coating. Microscopic characterization of mist-deposited QD-films has indicated that tight controls over the thickness and surface morphology of QD-layers can be achieved by tailoring the process variables. A QD-LED containing the mist-deposited emissive QD-layer was demonstrated with defect-free and uniform brightness. Furthermore, the technique of successive mist-deposition of multi-color QDs through a set of registered shallow masks was employed to create and investigate a 6×6 matrix of alternating pixels composed of 5nm-diameter CdSe(ZnS) QDs (green) and 8nm-diameter CdSe(ZnS) QDs (red) on a single substrate, suggesting the full capacity of mist-deposition technology in the future development of colorful QD-LED displays. It is expected that superior QD-LED performance can be achieved by optimizing the multiple parameters in the mist-deposition process. The demonstrated approach provides a solid platform for the next-stage development of multicolor QD-LED matrices by selectively mist-depositing multispectral QDs with shadow masks.

We present a new type of field-effect transistor (FET) based on films of PbSe nanocrystal in combination with ion gels as the dielectric material. Prior to applying ion gel, PbSe nanocrystal films were treated with hydrazine and stored under vacuum. Gating the film with a conventional SiO₂ dielectric, p-channel FETs were obtained with hole mobilities of 0.08 cm²/Vsec and current modulation of < 10² with applied gate voltage of -100 V to 100 V . The same nanocrystal film was then gated with ion gel dielectric, which has large specific capacitance (~ 30 μF/cm²). Ambipolar behavior was observed with improved device performance. In particular, the ambipolar transistor yielded electron and hole mobilities up to 1.9 and 0.2 cm²/Vsec, respectively and current modulations of 10² to 10³ with applied gate voltage of -2 V to 2 V.

It is a challenge to develop flat printable electronics based solely on inorganic materials as an active semiconductor. This could be the basis for flexible displays or electronic paper when the active material is processable from solution, shows very good adherence to flexible substrates and excellent physical performance. To meet these challenges any material considered requires a tuned set of functional properties. In general inorganic semiconductors are in advantage over organic materials as far as their physical performance is concerned. However, often processing and adherence to substrates is a problem with inorganic semiconductors. ZnO is available in various morphologies as transparent oxide, it is non toxic, inexpensive and is well known for its promising physical semiconductor properties. Despite a couple of recent reports on the deposition of zinc oxide thin layers in FET devices, processing from solution and conversion into the active FET channel electrode under fairly mild conditions is a great challenge. Chemical bath deposition techniques and sol-gel processes were mainly investigated in this regard. However, both techniques typically require either high processing temperatures (above 300°C) or long reaction times and are thus inappropriate for printing applications on flexible polymer based substrates under state of the art printing conditions. Processing temperatures well below 200° C are the goal for the formation of semiconducting inorganic thin films onto such substrates. We will report on synthesis of a molecular precursor, its use in the formation of naocrystalline ZnO and its conversion into self adhering films which can be deposited on plastics as well as more conventional substrates. Moreover the electrical performance of nanocrystalline ZnO derived therefrom is reported.