Design Principles of Colloidal Nanorod Heterostructures.

Anisotropic heterostructures of colloidal nanocrystals embed size-, shape-, and composition-dependent electronic structure within variable three-dimensional morphology, enabling intricate design of solution-processable materials with high performance and programmable functionality. The key to designing and synthesizing such complex materials lies in understanding the fundamental thermodynamic and kinetic factors that govern nanocrystal growth. In this review, nanorod heterostructures, the simplest of anisotropic nanocrystal heterostructures, are discussed with respect to their growth mechanisms. The effects of crystal structure, surface faceting/energies, lattice strain, ligand sterics, precursor reactivity, and reaction temperature on the growth of nanorod heterostructures through heteroepitaxy and cation exchange reactions are explored with currently known examples. Understanding the role of various thermodynamic and kinetic parameters enables the controlled synthesis of complex nanorod heterostructures that can exhibit unique tailored properties. Selected application prospects arising from such capabilities are then discussed.

Colloidal Multi-Dot Nanorods.

Colloidal nanorod heterostructures consisting of multiple quantum dots within a nanorod (n-DNRs, where n is the number of quantum dots within a nanorod) are synthesized with alternating segments of CdSe "dot" and CdS "rod" via solution heteroepitaxy. The reaction temperature, time dependent ripening, and asymmetry of the wurtzite lattice and the resulting anisotropy of surface ligand steric hindrance are exploited to vary the morphology of the growing quantum dot segments. The alternating CdSe and CdS growth steps can be easily repeated to increment the dot number in unidirectional or bidirectional growth regimes. As an initial exploration of electron occupation effects on their optical properties, asymmetric 2-DNRs consisting of two dots of different lengths and diameters are synthesized and are shown to exhibit a change in color and an unusual photoluminescence quantum yield increase upon photochemical doping.

pH Tunable Patterning of Quantum Dots.

Patterning of quantum dots (QDs) is essential for many, especially high-tech, applications. Here, pH tunable assembly of QDs over functional patterns prepared by electrohydrodynamic jet printing of poly(2-vinylpyridine) is presented. The selective adsorption of QDs from water dispersions is mediated by the electrostatic interaction between the ligand composed of 3-mercaptopropionic acid and patterned poly(2-vinylpyridine). The pH of the dispersion provides tunability at two levels. First, the adsorption density of QDs and fluorescence from the patterns can be modulated for pH > ≈4. Second, patterned features show unique type of disintegration resulting in randomly positioned features within areas defined by the printing for pH ≤ ≈4. The first capability is useful for deterministic patterning of QDs, whereas the second one enables hierarchically structured encoding of information by generating stochastic features of QDs within areas defined by the printing. This second capability is exploited for generating addressable security labels based on unclonable features. Through image analysis and feature matching algorithms, it is demonstrated that such patterns are unclonable in nature and provide a suitable platform for anti-counterfeiting applications. Collectively, the presented approach not only enables effective patterning of QDs, but also establishes key guidelines for addressable assembly of colloidal nanomaterials.

Coffee-Ring Mediated Thinning and Thickness-Dependent Dewetting Modes in Printed Polymer Droplets Coupled with Assembly of Quantum Dots for Anti-Counterfeiting.

Molecular transport processes in printed polymer droplets hold enormous importance for understanding wetting phenomena and designing systems in applications such as encoding, electronics, photonics, and sensing. This paper studies thickness-dependent dewetting modes that are activated by thermal annealing and driven by interfacial interactions within microscopically confined polymeric features. The printing of poly(2-vinylpyridine) is performed in a regime where coffee-ring effects lead to strong thinning of the central region of the deposit. Thermal annealing leads to two different modes of dewetting that depend on the thickness of the central region. Mode I refers to the formation of randomly positioned small features surrounded by large hemispherical ones located along the periphery of the printed features and occurs when the central regions are thin. Observed at large central thicknesses, Mode II mediates significant molecular transport from edges toward the center of the printed droplet with thermal annealing and forms a hemispherical feature from the initial ring-like deposit. The selective adsorption of red, green, and blue emitting quantum dots over the poly(2-vinylpyridine) results in photoluminescent patterns. The selective assembly of photoluminescent quantum dots over patterned surfaces leads to deterministic and stochastic features beneficial to creating security labels for anti-counterfeiting applications.

Improving photovoltaic performance of light-responsive double-heterojunction nanorod light-emitting diodes.

Double heterojunction nanorods enable both electroluminescence and light harvesting capabilities within the same device structure, providing a promising platform for energy-scavenging displays and related applications. However, the efficiency of the photovoltaic mode remains modest for useful power conversion and may be challenging to improve without sacrificing performance in electroluminescence. Through a facile on-film partial ligand exchange with benzenethiol integrated into the device fabrication step, we achieve an average of more than threefold increase in power conversion efficiency while maintaining the maximum external quantum efficiency and the maximum luminance in the LED mode. The improved photovoltaic performance is mainly due to the increase in the short circuit current, which we attribute to the enhanced charge separation afforded by the partial ligand exchange. The recovery of the photoluminescence lifetime under the forward bias suggests that the hole traps introduced by benzenethiols are filled prior to reaching the voltage at which light emission begins, allowing LED performance to be maintained and possibly improved.

Charging and Charged Species in Quantum Dot Light-Emitting Diodes.

Despite recent rapid advances in improving quantum dot light-emitting diodes, many fundamental aspects of the device operating mechanism remain unresolved. Through transient electroluminescence and time-resolved photoluminescence measurements, the effects of offset voltage on charging and charge transport are examined. First, capacitive charging occurs with a time constant of ∼500 ns, followed by electron transport through quantum dots with a mobility of ∼10-5 cm2 V-1 s-1. Hole injection then initiates an electroluminescence rise that is independent of offset voltage. The photoluminescence lifetime is also unaffected by the offset voltage, indicating no injection of charges into the quantum dots or on their surfaces prior to the voltage pulse. A slower equilibration to steady-state electroluminescence is dependent on the offset voltage, indicative of another charging process. Elemental mapping shows that ZnO deposition from solution can lead to the diffusion of charged species into the quantum dot layer, which may cause the slower process.

Highly luminescent double-heterojunction nanorods.

Anisotropic shape and band structure engineered into double-heterojunction nanorods (DHNRs) can improve and impart new optical/optoelectronic capabilities in colloidal quantum dot-based devices. However, the photoluminescence quantum yield of DHNRs, which is significantly lower than the near-ideal limit recently achieved in the state-of-the-art core/shell quantum dots, remains as their main limitation. Here, we examine how the photoluminescence of CdS/CdSe/ZnSe DHNRs is affected by (1) the length of the CdS seed nanorods, (2) the rod and tip diameter dependent variations in band offset, and (3) the CdSe-like islands on the sides of DHNRs that can result as a side-product of ZnSe shell growth. By understanding and optimizing these three key parameters, we demonstrate an improvement in the photoluminescence quantum yield up to 93% (up to 62% for excitation above CdS bandgap) while retaining useful shape anisotropy and the band structure design in DHNRs.

Metal Oleate Induced Etching and Growth of Semiconductor Nanocrystals, Nanorods, and Their Heterostructures.

Unexpected etching of nanocrystals, nanorods, and their heterostructures by one of the most commonly used metal precursors, metal oleates, is reported. Zn oleate is shown to etch CdS nanorods anisotropically, where the length decreases without a significant change in the diameter. Sodium oleate enhances the etch rate, whereas oleic acid alone does not cause etching, indicating the importance of the countercation on the rate of oleate induced etching. Subsequent addition of Se precursors to the partially etched nanorods in Zn oleate solution can lead to epitaxial growth of CdSe particles rather than the expected ZnSe growth, despite an excess amount of Zn precursors being present. The composition of this epitaxial growth can be varied from CdSe to ZnSe, depending on the amount of excess oleic acid or the reaction temperature. Similar tuning of composition can be observed when starting with collinear CdSe/CdS/CdSe rod/rod/rod heterostructures and spherical CdS (or CdSe/CdS core/shell) nanocrystals. Conversion of collinear rod/rod/rod structures to barbells and interesting rod growth from nearly spherical particles among other structures can also result due to the initial etching effect of metal oleates. These observations have important implications on our understanding of nanocrystal heterostructure synthesis and open up new routes to varying the composition and morphology of these materials.

Cu2 S/ZnS Heterostructured Nanorods: Cation Exchange vs. Solution-Liquid-Solid-like Growth.

Cu2 S/ZnS heterostructured nanorods (HNRs) with uncommon morphologies are achieved through single-pot and multi-batch synthetic strategies. In both cases, Cu2 S NRs form first, which then undergo partial cation exchange and solution-liquid-solid (SLS)-like growth catalyzed by the remaining Cu2 S parts of the NRs. The location and the volume of ZnS achieved through partial cation exchange control the size of the Cu2 S catalysts, which in turn determine whether tapered rod-rod, body/tail, or barbell-like structure results from subsequent SLS-like growth. Concurrent cation exchange can sometimes cause Cu2 S catalysts to be lost during SLS-like growth, leading to further diversity in achievable morphologies of Cu2 S/ZnS HNRs. Additional insights are gained on how parameters such as Zn precursor, ligand choice, and concentration alter cation exchange and SLS-like growth steps.

Solution-Processed Transistors Using Colloidal Nanocrystals with Composition-Matched Molecular "Solders": Approaching Single Crystal Mobility.

Crystalline silicon-based complementary metal-oxide-semiconductor transistors have become a dominant platform for today's electronics. For such devices, expensive and complicated vacuum processes are used in the preparation of active layers. This increases cost and restricts the scope of applications. Here, we demonstrate high-performance solution-processed CdSe nanocrystal (NC) field-effect transistors (FETs) that exhibit very high carrier mobilities (over 400 cm(2)/(V s)). This is comparable to the carrier mobilities of crystalline silicon-based transistors. Furthermore, our NC FETs exhibit high operational stability and MHz switching speeds. These NC FETs are prepared by spin coating colloidal solutions of CdSe NCs capped with molecular solders [Cd2Se3](2-) onto various oxide gate dielectrics followed by thermal annealing. We show that the nature of gate dielectrics plays an important role in soldered CdSe NC FETs. The capacitance of dielectrics and the NC electronic structure near gate dielectric affect the distribution of localized traps and trap filling, determining carrier mobility and operational stability of the NC FETs. We expand the application of the NC soldering process to core-shell NCs consisting of a III-V InAs core and a CdSe shell with composition-matched [Cd2Se3](2-) molecular solders. Soldering CdSe shells forms nanoheterostructured material that combines high electron mobility and near-IR photoresponse.

High-resolution patterns of quantum dots formed by electrohydrodynamic jet printing for light-emitting diodes.

Here we demonstrate materials and operating conditions that allow for high-resolution printing of layers of quantum dots (QDs) with precise control over thickness and submicron lateral resolution and capabilities for use as active layers of QD light-emitting diodes (LEDs). The shapes and thicknesses of the QD patterns exhibit systematic dependence on the dimensions of the printing nozzle and the ink composition in ways that allow nearly arbitrary, systematic control when exploited in a fully automated printing tool. Homogeneous arrays of patterns of QDs serve as the basis for corresponding arrays of QD LEDs that exhibit excellent performance. Sequential printing of different types of QDs in a multilayer stack or in an interdigitated geometry provides strategies for continuous tuning of the effective, overall emission wavelengths of the resulting QD LEDs. This strategy is useful to efficient, additive use of QDs for wide ranging types of electronic and optoelectronic devices.

Medium-scale carbon nanotube thin-film integrated circuits on flexible plastic substrates.

The ability to form integrated circuits on flexible sheets of plastic enables attributes (for example conformal and flexible formats and lightweight and shock resistant construction) in electronic devices that are difficult or impossible to achieve with technologies that use semiconductor wafers or glass plates as substrates. Organic small-molecule and polymer-based materials represent the most widely explored types of semiconductors for such flexible circuitry. Although these materials and those that use films or nanostructures of inorganics have promise for certain applications, existing demonstrations of them in circuits on plastic indicate modest performance characteristics that might restrict the application possibilities. Here we report implementations of a comparatively high-performance carbon-based semiconductor consisting of sub-monolayer, random networks of single-walled carbon nanotubes to yield small- to medium-scale integrated digital circuits, composed of up to nearly 100 transistors on plastic substrates. Transistors in these integrated circuits have excellent properties: mobilities as high as 80 cm(2) V(-1) s(-1), subthreshold slopes as low as 140 m V dec(-1), operating voltages less than 5 V together with deterministic control over the threshold voltages, on/off ratios as high as 10(5), switching speeds in the kilohertz range even for coarse (approximately 100-microm) device geometries, and good mechanical flexibility-all with levels of uniformity and reproducibility that enable high-yield fabrication of integrated circuits. Theoretical calculations, in contexts ranging from heterogeneous percolative transport through the networks to compact models for the transistors to circuit level simulations, provide quantitative and predictive understanding of these systems. Taken together, these results suggest that sub-monolayer films of single-walled carbon nanotubes are attractive materials for flexible integrated circuits, with many potential areas of application in consumer and other areas of electronics.

Tunable carrier type and density in graphene/PbZr0.2Ti0.8O3 hybrid structures through ferroelectric switching.

Bidirectional interdependency between graphene doping level and ferroelectric polarization is demonstrated in graphene/PbZr0.2Ti0.8O3 hybrid structures. The polarization of the PbZr0.2Ti0.8O3 can be effectively switched with graphene electrodes and can in turn alter carrier type and density in the graphene. A complete reversal of the current-voltage hysteresis direction is observed in the graphene when external environmental factors are minimized, converting p-type graphene into n-type with an estimated carrier density change as large as ~10(13) cm(-2). Nonvolatility and reversibility are also demonstrated.

A high temperature in situ optical probe for colloidal nanocrystal synthesis.

We report on the fabrication and utilization of a robust high-temperature (>300 °C), adjustable-path-length, vacuum-tolerant, configurable, in situ optical probe, which interfaces with widely used chemical glassware via a 14/20 ground glass joint. This probe allows for high-speed reaction monitoring of colloidal semiconductor nanocrystal solutions at temperatures that were previously inaccessible. We demonstrate this capability by monitoring the hot-injection synthesis of CdSe quantum dots via UV-Vis absorption spectroscopy at 380 °C with a time resolution of ∼10 ms, with the primary limitation being the acquisition and data saving rate of the commercial spectrometer used. We further demonstrate that this probe can also be used for in situ photoluminescence measurements. This system is generally applicable to harsh solution environments where optical monitoring of reaction progress is desirable and/or necessary.

Mechanism of morphology variations in colloidal CuGaS2 nanorods.

Cu2-x S nanocrystals can serve as templates and intermediates in the synthesis of a wide range of nanocrystals through seeded growth, cation exchange, and/or catalytic growth. This versatility can facilitate and accelerate the search for environmentally benign nanocrystals of high performance with variable shapes, sizes, and composition. However, expanding the compositional space via Cu2-x S nanocrystals while achieving necessary uniformity requires an improved understanding of the growth mechanisms. Herein we address several unusual and previously unexplained aspects of the growth of CuGaS2 nanorods from Cu2-x S seeds as an example. In particular, we address the origin of the diverse morphologies which manifest from a relatively homogeneous starting mixture. We find that CuGaS2 nanorods start as Cu2-x S/CuGaS2 Janus particles, the majority of which have a {101̄2}/{101̄2} interface that helps to minimize lattice strain. We propose a mechanism that involves concurrent seed growth and cation exchange (CSC), where epitaxial growth of the Cu2-x S seed, rather than the anticipated catalytic or seeded growth of CuGaS2, occurs along with cation exchange that converts growing Cu2-x S to CuGaS2. This mechanism can explain the incorporation of the large number of anions needed to account for the order-of-magnitude volume increase upon CuGaS2 rod growth (which cannot be accounted for by the commonly assumed catalytic growth mechanism) and variations in morphology, including the pervasive tapering and growth direction change. Insights from the CSC growth mechanism also help to explain a previously puzzling phenomenon of regioselective nucleation of CuInSe2 on kinked CuGaS2 nanorods.

Accelerated screening of colloidal nanocrystals using artificial neural network-assisted autonomous flow reactor technology.

Colloidal semiconductor nanocrystals with tunable optical and electronic properties are opening up exciting opportunities for high-performance optoelectronics, photovoltaics, and bioimaging applications. Identifying the optimal synthesis conditions and screening of synthesis recipes in search of efficient synthesis pathways to obtain nanocrystals with desired optoelectronic properties, however, remains one of the major bottlenecks for accelerated discovery of colloidal nanocrystals. Conventional strategies, often guided by limited understanding of the underlying mechanisms remain expensive in both time and resources, thus significantly impeding the overall discovery process. In response, an autonomous experimentation platform is presented as a viable approach for accelerated synthesis screening and optimization of colloidal nanocrystals. Using a machine-learning-based predictive synthesis approach, integrated with automated flow reactor and inline spectroscopy, indium phosphide nanocrystals are autonomously synthesized. Their polydispersity for different target absorption wavelengths across the visible spectrum is simultaneously optimized during the autonomous experimentation, while utilizing minimal self-driven experiments (less than 50 experiments within 2 days). Starting with no-prior-knowledge of the synthesis, an ensemble neural network is trained through autonomous experiments to accurately predict the reaction outcome across the entire synthesis parameter space. The predicted parameter space map also provides new nucleation-growth kinetic insights to achieve high monodispersity in size of colloidal nanocrystals.

Anisotropic strain-induced curvature in type-II CdSe/CdTe nanorod heterostructures.

Type-II band-offset CdSe/CdTe nanorod heterostructures with curved and linear shapes have been synthesized and examined with atomic-resolution transmission electron microscopy techniques. Strain from growth of larger-lattice CdTe partly on the sides of CdSe nanorod seeds is shown to lead to an overall curvature in the rods. Lattice expansion from the inner to the outer portion of the curved region exceeds the expected lattice mismatch between the two materials because of the buildup of an unusual compressive strain in the CdSe. In contrast, exclusive tip growth results in linear barbell-shaped heterostructures that do not exhibit strain-induced curvature. The ability to vary the anisotropic lattice strain should allow control over the underlying electronic structure, providing new approaches to directing photogenerated carriers that may facilitate incorporation of nanorod heterostructures in various energy applications.

DC modeling and the source of flicker noise in passivated carbon nanotube transistors.

DC and intrinsic low-frequency noise properties of p-channel depletion-mode carbon nanotube field effect transistors (CNT-FETs) are investigated. To characterize the intrinsic noise properties, a thin atomic layer deposited (ALD) HfO(2) gate dielectric is used as a passivation layer to isolate CNT-FETs from environmental factors. The ALD HfO(2) gate dielectric in these high-performance top-gated devices is instrumental in attaining hysteresis-free current-voltage characteristics and minimizes low-frequency noise. Under small drain-source voltage, the carriers in the CNT channel are modulated by the gate electrode and the intrinsic 1/f noise is found to be correlated with charge trapping/detrapping from the oxide substrate as expected. When thermionic emission is the dominant carrier transport mechanism in CNT-FETs under large drain-source voltages, the excess 1/f noise is attributed to the noise stemming from metal-CNT Schottky barrier contacts as revealed by the measurements.

Role of covalent defects on phonon softening in metallic carbon nanotubes.

We have examined how electrical characteristics and charging dependent Raman G-band phonon softening in individual metallic carbon nanotubes are influenced by covalent defects. In addition to decreasing electrical conductance with increasing on/off current ratio eventually leading to semiconducting behavior, adding covalent defects reduces the degree of softening and broadening of longitudinal optical (LO) phonon mode of the G-band near the charge neutrality point where the bands cross. On the other hand, the transverse optical (TO) mode softening is enhanced by defects. Implications on the interpretation of Raman G-band phonon softening and on utilizing Raman spectroscopy to examine covalent functionalization are discussed.

Manifestation of Kohn anomaly in 1/f fluctuations in metallic carbon nanotubes.

Low-frequency noise in metallic single walled carbon nanotubes is shown to be strongly dependent on the Fermi level position and the applied electric field across the nanotube. Resonance-like enhancement observed near optical phonon energy only when the Fermi level lies near the Dirac point is correlated to Raman G-band softening and broadening. The results suggest that the competition between zone-center and zone-boundary phonon scattering is the underlying origin of the large enhancement and resonance-like behavior of 1/f noise.