Nanocrystal phononics.

Colloidal nanocrystals are successfully used as nanoscale building blocks for creating hierarchical solids with structures that range from amorphous networks to sophisticated periodic superlattices. Recently, it has been observed that these superlattices exhibit collective vibrations, which stem from the correlated motion of the nanocrystals, with their surface-bound ligands acting as molecular linkers. In this Perspective, we describe the work so far on collective vibrations in nanocrystal solids and their as-of-yet untapped potential for phononic applications. With the ability to engineer vibrations in the hypersonic regime through the choice of nanocrystal and linker composition, as well as by controlling their size, shape and chemical interactions, such superstructures offer new opportunities for phononic crystals, acoustic metamaterials and optomechanical systems.

Sol-Gel Barium Titanate Nanohole Array as a Nonlinear Metasurface and a Photonic Crystal.

The quest of a nonlinear optical material that can be easily nanostructured over a large surface area is still ongoing. Here, we demonstrate a nanoimprinted nonlinear barium titanate 2D nanohole array that shows the optical properties of a 2D photonic crystal and a metasurface, depending on the direction of the optical axis. The challenge of nanostructuring the inert metal-oxide is resolved by direct soft nanoimprint lithography with sol-gel derived barium titanate enabling critical dimensions of 120 nm with aspect ratios of five. The nanohole array exhibits a photonic bandgap in the infrared range when probed along the slab axis, while lattice resonant states are observed in out-of-plane transmission configuration. The enhanced light-matter interaction from the resonant structure enables to increase in the second-harmonic generation in the near-ultraviolet by a factor of 18 illustrating the potential in the flexible fabrication technique for barium titanate photonic devices.

Gas-sieving zeolitic membranes fabricated by condensation of precursor nanosheets.

The synthesis of molecular-sieving zeolitic membranes by the assembly of building blocks, avoiding the hydrothermal treatment, is highly desired to improve reproducibility and scalability. Here we report exfoliation of the sodalite precursor RUB-15 into crystalline 0.8-nm-thick nanosheets, that host hydrogen-sieving six-membered rings (6-MRs) of SiO4 tetrahedra. Thin films, fabricated by the filtration of a suspension of exfoliated nanosheets, possess two transport pathways: 6-MR apertures and intersheet gaps. The latter were found to dominate the gas transport and yielded a molecular cutoff of 3.6 Å with a H2/N2 selectivity above 20. The gaps were successfully removed by the condensation of the terminal silanol groups of RUB-15 to yield H2/CO2 selectivities up to 100. The high selectivity was exclusively from the transport across 6-MR, which was confirmed by a good agreement between the experimentally determined apparent activation energy of H2 and that computed by ab initio calculations. The scalable fabrication and the attractive sieving performance at 250-300 °C make these membranes promising for precombustion carbon capture.

Soft surfaces of nanomaterials enable strong phonon interactions.

Phonons and their interactions with other phonons, electrons or photons drive energy gain, loss and transport in materials. Although the phonon density of states has been measured and calculated in bulk crystalline semiconductors, phonons remain poorly understood in nanomaterials, despite the increasing prevalence of bottom-up fabrication of semiconductors from nanomaterials and the integration of nanometre-sized components into devices. Here we quantify the phononic properties of bottom-up fabricated semiconductors as a function of crystallite size using inelastic neutron scattering measurements and ab initio molecular dynamics simulations. We show that, unlike in microcrystalline semiconductors, the phonon modes of semiconductors with nanocrystalline domains exhibit both reduced symmetry and low energy owing to mechanical softness at the surface of those domains. These properties become important when phonons couple to electrons in semiconductor devices. Although it was initially believed that the coupling between electrons and phonons is suppressed in nanocrystalline materials owing to the scarcity of electronic states and their large energy separation, it has since been shown that the electron-phonon coupling is large and allows high energy-dissipation rates exceeding one electronvolt per picosecond (refs 10-13). Despite detailed investigations into the role of phonons in exciton dynamics, leading to a variety of suggestions as to the origins of these fast transition rates and including attempts to numerically calculate them, fundamental questions surrounding electron-phonon interactions in nanomaterials remain unresolved. By combining the microscopic and thermodynamic theories of phonons and our findings on the phononic properties of nanomaterials, we are able to explain and then experimentally confirm the strong electron-phonon coupling and fast multi-phonon transition rates of charge carriers to trap states. This improved understanding of phonon processes permits the rational selection of nanomaterials, their surface treatments, and the design of devices incorporating them.

Compact Mid-Infrared Gas Sensing Enabled by an All-Metamaterial Design.

The miniaturization of mid-infrared optical gas sensors has great potential to make the "fingerprint region" between 2 and 10 µm accessible to a variety of cost-sensitive applications ranging from medical technology to atmospheric sensing. Here we demonstrate a gas sensor concept that achieves a 30-fold reduction in absorption volume compared to conventional gas sensors by using plasmonic metamaterials as on-chip optical filters. Integrating metamaterials into both the emitter and the detector cascades their individual filter functions, yielding a narrowband spectral response tailored to the absorption band of interest, here CO2. Simultaneously, the metamaterials' angle-independence is maintained, enabling an optically efficient, millimeter-scale cavity. With a CO2 sensitivity of 22.4 ± 0.5 ppm·Hz-0.5, the electrically driven prototype already performs at par with much larger commercial devices while consuming 80% less energy per measurement. The all-metamaterial sensing concept offers a path toward more compact and energy-efficient mid-infrared gas sensors without trade-offs in sensitivity or robustness.

Phonon-Mediated and Weakly Size-Dependent Electron and Hole Cooling in CsPbBr3 Nanocrystals Revealed by Atomistic Simulations and Ultrafast Spectroscopy.

We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over ∼10 ps) of "warm" electrons in the vicinity (within ∼0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps-1 carrier-1, increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intraband transitions driven by strong and size-dependent electron-phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers.

Nanocrystal Quantum Dot Devices: How the Lead Sulfide (PbS) System Teaches Us the Importance of Surfaces.

Semiconducting thin films made from nanocrystals hold potential as composite hybrid materials with new functionalities. With nanocrystal syntheses, composition can be controlled at the sub-nanometer level, and, by tuning size, shape, and surface termination of the nanocrystals as well as their packing, it is possible to select the electronic, phononic, and photonic properties of the resulting thin films. While the ability to tune the properties of a semiconductor from the atomistic- to macro-scale using solution-based techniques presents unique opportunities, it also introduces challenges for process control and reproducibility. In this review, we use the example of well-studied lead sulfide (PbS) nanocrystals and describe the key advances in nanocrystal synthesis and thin-film fabrication that have enabled improvement in performance of photovoltaic devices. While research moves forward with novel nanocrystal materials, it is important to consider what decades of work on PbS nanocrystals has taught us and how we can apply these learnings to realize the full potential of nanocrystal solids as highly flexible materials systems for functional semiconductor thin-film devices. One key lesson is the importance of controlling and manipulating surfaces.

Simulating nanocrystal-based solar cells: A lead sulfide case study.

Nanocrystal-based solar cells are promising candidates for next generation photovoltaic applications; however, the most recent improvements to the device chemistry and architecture have been mostly trial-and-error based advancements. Due to complex interdependencies among parameters, determining factors that limit overall solar cell efficiency are not trivial. Furthermore, many of the underlying chemical and physical parameters of nanocrystal-based solar cells have only recently been understood and quantified. Here, we show that this new understanding of interfaces, transport, and origin of trap states in nanocrystal-based semiconductors can be integrated into simulation tools, based on 1D drift-diffusion models. Using input parameters measured in independent experiments, we find excellent agreement between experimentally measured and simulated PbS nanocrystal solar cell behavior without having to fit any parameters. We then use this simulation to understand the impact of interfaces, charge carrier mobility, and trap-assisted recombination on nanocrystal performance. We find that careful engineering of the interface between the nanocrystals and the current collector is crucial for an optimal open-circuit voltage. We also show that in the regime of trap-state densities found in PbS nanocrystal solar cells (∼1017 cm-3), device performance exhibits strong dependence on the trap state density, explaining the sensitivity of power conversion efficiency to small changes in nanocrystal synthesis and nanocrystal thin-film deposition that has been reported in the literature. Based on these findings, we propose a systematic approach to nanocrystal solar cell optimization. Our method for incorporating parameters into simulations presented and validated here can be adopted to speed up the understanding and development of all types of nanocrystal-based solar cells.

Tuning Electron-Phonon Interactions in Nanocrystals through Surface Termination.

We perform ab initio molecular dynamics on experimentally relevant-sized lead sulfide (PbS) nanocrystals (NCs) constructed with thiol or Cl, Br, and I anion surfaces to determine their vibrational and dynamic electronic structure. We show that electron-phonon interactions can explain the large thermal broadening and fast carrier cooling rates experimentally observed in Pb-chalcogenide NCs. Furthermore, our simulations reveal that electron-phonon interactions are suppressed in halide-terminated NCs due to reduction of both the thermal displacement of surface atoms and the spatial overlap of the charge carriers with these large atomic vibrations. This work shows how surface engineering, guided by simulations, can be used to systematically control carrier dynamics.

Quantification of deep traps in nanocrystal solids, their electronic properties, and their influence on device behavior.

We implement three complementary techniques to quantify the number, energy, and electronic properties of trap states in nanocrystal (NC)-based devices. We demonstrate that, for a given technique, the ability to observe traps depends on the Fermi level position, highlighting the importance of a multitechnique approach that probes trap coupling to both the conduction and the valence bands. We then apply our protocol for characterizing traps to quantitatively explain the measured performances of PbS NC-based solar cells.

Phase-Controlled Synthesis and Phase-Change Properties of Colloidal Cu-Ge-Te Nanoparticles.

Phase-change memory (PCM) technology has recently attracted a vivid interest for neuromorphic applications, in-memory computing, and photonic integration due to the tunable refractive index and electrical conductivity between the amorphous and crystalline material states. Despite this, it is increasingly challenging to scale down the device dimensions of conventionally sputtered PCM memory arrays, restricting the implementation of PCM technology in mass applications such as consumer electronics. Here, we report the synthesis and structural study of sub-10 nm Cu-Ge-Te (CGT) nanoparticles as suitable candidates for low-cost and ultrasmall PCM devices. We show that our synthesis approach can accurately control the structure of the CGT colloids, such as composition-tuned CGT amorphous nanoparticles as well as crystalline CGT nanoparticles with trigonal α-GeTe and tetragonal Cu2GeTe3 phases. In situ characterization techniques such as high-temperature X-ray diffraction and X-ray absorption spectroscopy reveal that Cu doping in GeTe improves the thermal properties and amorphous phase stability of the nanoparticles, in addition to nanoscale effects, which enhance the nonvolatility characteristics of CGT nanoparticles even further. Moreover, we demonstrate the thin-film fabrication of CGT nanoparticles and characterize their optical properties with spectroscopic ellipsometry measurements. We reveal that CGT nanoparticle thin films exhibit a negative reflectivity change and have good reflectivity contrast in the near-IR spectrum. Our work promotes the possibility to use PCM in nanoparticle form for applications such as electro-optical switching devices, metalenses, reflectivity displays, and phase-change IR devices.

Phase-Change Memory from Molecular Tellurides.

Phase-change memory (PCM) is an emerging memory technology based on the resistance contrast between the crystalline and amorphous states of a material. Further development and realization of PCM as a mainstream memory technology rely on innovative materials and inexpensive fabrication methods. Here, we propose a generalizable and scalable solution-processing approach to synthesize phase-change telluride inks in order to meet demands for high-throughput material screening, increased energy efficiency, and advanced device architectures. Bulk tellurides, such as Sb2Te3, GeTe, Sc2Te3, and TiTe2, are dissolved and purified to obtain inks of molecular metal telluride complexes. This allowed us to unlock a wide range of solution-processed ternary tellurides by the simple mixing of binary inks. We demonstrate accurate and quantitative composition control, including prototype materials (Ge-Sb-Te) and emerging rare-earth-metal telluride-doped materials (Sc-Sb-Te). Spin-coating and annealing convert ink formulations into high-quality, phase-pure telluride films with preferred orientation along the (00l) direction. Deposition engineering of liquid tellurides enables thickness-tunable films, infilling of nanoscale vias, and film preparation on flexible substrates. Finally, we demonstrate cyclable and non-volatile prototype memory devices, achieving performance indicators such as resistance contrast and low reset energy on par with state-of-the-art sputtered PCM layers.

Unravelling the amorphous structure and crystallization mechanism of GeTe phase change memory materials.

The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable 'ideal glass' state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature.

Disorder and Halide Distributions in Cesium Lead Halide Nanocrystals as Seen by Colloidal 133Cs Nuclear Magnetic Resonance Spectroscopy.

Colloidal nuclear magnetic resonance (cNMR) spectroscopy on inorganic cesium lead halide nanocrystals (CsPbX3 NCs) is found to serve for noninvasive characterization and quantification of disorder within these structurally soft and labile particles. In particular, we show that 133Cs cNMR is highly responsive to size variations from 3 to 11 nm or to altering the capping ligands on the surfaces of CsPbX3 NCs. Distinct 133Cs signals are attributed to the surface and core NC regions. Increased heterogeneous broadening of 133Cs signals, observed for smaller NCs as well as for long-chain zwitterionic capping ligands (phosphocholines, phosphoethanol(propanol)amine, and sulfobetaines), can be attributed to more significant surface disorder and multifaceted surfaces (truncated cubes). On the contrary, capping with dimethyldidodecylammonium bromide (DDAB) successfully reduces signal broadening owing to better surface passivation and sharper (001)-bound cuboid shape. DFT calculations on various sizes of NCs corroborate the notion that the surface disorder propagates over several octahedral layers. 133Cs NMR is a sensitive probe for studying halide gradients in mixed Br/Cl NCs, indicating bromide-rich surfaces and chloride-rich cores. On the contrary, mixed Br/I NCs exhibit homogeneous halide distributions.

Structural Ordering in Ultrasmall Multicomponent Chalcogenides: The Case of Quaternary Cu-Zn-In-Se Nanocrystals.

The compositional tunability of non-isovalent multicomponent chalcogenide thin films and the extent of atomic ordering of their crystal structure is key to the performance of many modern technologies. In contrast, the effects of ordering are rarely studied for quantum-confined materials, such as colloidal nanocrystals. In this paper, the possibilities around composition tunability and atomic ordering are explored in ultrasmall ternary and quaternary quantum dots, taking I-III-VI-group Cu-Zn-In-Se semiconductor as a case study. A quantitative synthesis for 3.3 nm quaternary chalcogenide nanocrystals is developed and shown that cation and cationic vacancy ordering can be achieved in these systems consisting of only 100s of atoms. Combining experiment and theoretical calculations, the relationship between structural ordering and optical properties of the materials are demonstrated. It is found that the arrangement and ordering of cationic sublattice plays an important role in the luminescent efficiency. Specifically, the concentration of Cu-vacancy couples in the nanocrystal correlates with luminescence quantum yield, while structure ordering increases the occurrence of such optically active Cu-vacancy units. On the flip side, the detrimental impact of cationic site disorder in I-III-VI nanocrystals can be mitigated by introducing a cation of intermediate valence, such as Zn (II).

Coupling to octahedral tilts in halide perovskite nanocrystals induces phonon-mediated attractive interactions between excitons.

Understanding the origin of electron-phonon coupling in lead halide perovskites is key to interpreting and leveraging their optical and electronic properties. Here we show that photoexcitation drives a reduction of the lead-halide-lead bond angles, a result of deformation potential coupling to low-energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump-electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr3. Our results indicate a stronger coupling in FAPbBr3 than CsPbBr3. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength that scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in lead halide perovskites.

Synthesis and Electronic Structure of Mid-Infrared Absorbing Cu3SbSe4 and CuxSbSe4 Nanocrystals.

Aliovalent I-V-VI semiconductor nanocrystals are promising candidates for thermoelectric and optoelectronic applications. Famatinite Cu3SbSe4 stands out due to its high absorption coefficient and narrow band gap in the mid-infrared spectral range. This paper combines experiment and theory to investigate the synthesis and electronic structure of colloidal CuxSbSe4 nanocrystals. We achieve predictive composition control of size-uniform CuxSbSe4 (x = 1.9-3.4) nanocrystals. Density functional theory (DFT)-parametrized tight-binding simulations on nanocrystals show that the more the Cu-vacancies, the wider the band gap of CuxSbSe4 nanocrystals, a trend which we also confirm experimentally via FTIR spectroscopy. We show that SbCu antisite defects can create mid-gap states, which may give rise to sub-bandgap absorption. This work provides a detailed study of CuxSbSe4 nanocrystals and highlights the potential opportunities as well as challenges for their application in infrared devices.

Highly Responsive Mid-Infrared Metamaterial Enhanced Heterostructure Photodetector Formed out of Sintered PbSe/PbS Colloidal Quantum Dots.

Efficient and simple-to-fabricate light detectors in the mid infrared (MIR) spectral range are of great importance for various applications in existing and emerging technologies. Here, we demonstrate compact and efficient photodetectors operating at room temperature in a wavelength range of 2710-4250 nm with responsivities as high as 375 and 4 A/W. Key to the high performance is the combination of a sintered colloidal quantum dot (CQD) lead selenide (PbSe) and lead sulfide (PbS) heterojunction photoconductor with a metallic metasurface perfect absorber. The combination of this photoconductor stack with the metallic metasurface perfect absorber provides an overall ∼20-fold increase of the responsivity compared against reference sintered PbSe photoconductors. More precisely, the introduction of a PbSe/PbS heterojunction increases the responsivity by a factor of ∼2 and the metallic metasurface enhances the responsivity by an order of magnitude. The metasurface not only enhances the light-matter interaction but also acts as an electrode to the detector. Furthermore, fabrication of our devices relies on simple and inexpensive methods. This is in contrast to most of the currently available (state-of-the-art) MIR photodetectors that rely on rather expensive as well as nontrivial fabrication technologies that often require cooling for efficient operation.

Colloidal Ternary Telluride Quantum Dots for Tunable Phase Change Optics in the Visible and Near-Infrared.

A structural change between amorphous and crystalline phase provides a basis for reliable and modular photonic and electronic devices, such as nonvolatile memory, beam steerers, solid-state reflective displays, or mid-IR antennas. In this paper, we leverage the benefits of liquid-based synthesis to access phase-change memory tellurides in the form of colloidally stable quantum dots. We report a library of ternary MxGe1-xTe colloids (where M is Sn, Bi, Pb, In, Co, Ag) and then showcase the phase, composition, and size tunability for Sn-Ge-Te quantum dots. Full chemical control of Sn-Ge-Te quantum dots permits a systematic study of structural and optical properties of this phase-change nanomaterial. Specifically, we report composition-dependent crystallization temperature for Sn-Ge-Te quantum dots, which is notably higher compared to bulk thin films. This gives the synergistic benefit of tailoring dopant and material dimension to combine the superior aging properties and ultrafast crystallization kinetics of bulk Sn-Ge-Te, while improving memory data retention due to nanoscale size effects. Furthermore, we discover a large reflectivity contrast between amorphous and crystalline Sn-Ge-Te thin films, exceeding 0.7 in the near-IR spectrum region. We utilize these excellent phase-change optical properties of Sn-Ge-Te quantum dots along with liquid-based processability for nonvolatile multicolor images and electro-optical phase-change devices. Our colloidal approach for phase-change applications offers higher customizability of materials, simpler fabrication, and further miniaturization to the sub-10 nm phase-change devices.

Electroluminescence from nanoscale materials via field-driven ionization.

The high degree of morphological and energetic disorder inherent to many nanosized materials places limitations on charge injection into and transport rates through thin films of these materials. We demonstrate electroluminescence achieved by local generation of charge that eliminates the need for injection of charge carriers from the device electrodes. We show electroluminescence from thin films of nanoscale materials that do not support direct current excitation and suggest a mechanism for the charge generation and electroluminescence that is consistent with our time-averaged and time-resolved observations.