Programmable Interfacial Band Configuration in WS2/Bi2O2Se Heterojunctions.

van der Waals heterojunctions based on transition-metal dichalcogenides (TMDs) offer advanced strategies for manipulating light-emitting and light-harvesting behaviors. A crucial factor determining the light-material interaction is in the band alignment at the heterojunction interface, particularly the distinctions between type-I and type-II alignments. However, altering the band alignment from one type to another without changing the constituent materials is exceptionally difficult. Here, utilizing Bi2O2Se with a thickness-dependent band gap as a bottom layer, we present an innovative strategy for engineering interfacial band configurations in WS2/Bi2O2Se heterojunctions. In particular, we achieve tuning of the band alignment from type-I (Bi2O2Se straddling WS2) to type-II and finally to type-I (WS2 straddling Bi2O2Se) by increasing the thickness of the Bi2O2Se bottom layer from monolayer to multilayer. We verified this band architecture conversion using steady-state and transient spectroscopy as well as density functional theory calculations. Using this material combination, we further design a sophisticated band architecture incorporating both type-I (WS2 straddles Bi2O2Se, fluorescence-quenched) and type-I (Bi2SeO5 straddles WS2, fluorescence-recovered) alignments in one sample through focused laser beam (FLB). By programming the FLB trajectory, we achieve a predesigned localized fluorescence micropattern on WS2 without changing its intrinsic atomic structure. This effective band architecture design strategy represents a significant leap forward in harnessing the potential of TMD heterojunctions for multifunctional photonic applications.

Identifying Luminescent Boron Vacancies in h-BN Generated Using Controlled He+ Ion Irradiation.

The defect emission from h-BN at 1.55 eV is interesting as it enables optical readout of spins. It is necessary to identify the nature of the relevant point defects for its controlled introduction. However, it is challenging to engineer point defects in h-BN without changing the local atomic structure. Here, we controllably introduce boron vacancies in h-BN using an ultrahigh spatial resolution and low-energy He+ ion beam. By optimizing the He+ ion irradiation conditions, we control the quantity and location of defects spatially and along the depth of h-BN to achieve a robust photoluminescence emission at 1.55 eV from 10 K to room temperature. We show that as-generated defects activate an additional Raman mode at 1295 cm-1. Electron energy loss spectroscopy confirms introduction of boron vacancies without modification of the local h-BN crystal structure. Our results provide a deterministic strategy to create scalable boron vacancy emitters in h-BN for quantum photonics.

Freestanding High-Resolution Quantum Dot Color Converters with Small Pixel Sizes.

Designing the next generation of high-resolution displays requires high pixel density per area and small pixel sizes without compromising the optical quality. Quantum dots (QDs) have been demonstrated as a promising material system for down-conversion of blue emission as they provide pure colors on the wide color gamut. However, for high color-conversion efficiency, the required QD film thickness has not been compatible with small pixel sizes. In this work, we develop a new type of freestanding QD-based color converter for efficient optical down-conversion from inorganic blue light-emitting diodes (LEDs) in a color-by-blue configuration. CdSe/ZnS core-shell QDs in a UV-curable polymer matrix are encapsulated within cavities formed by patterning and bonding a pair of patterned quartz substrates. By controlling the required QD thickness and the pixel size independently, we demonstrate freestanding monochrome red and green converters with small pixel sizes down to 5 × 5 µm2 and a high resolution of >3600 ppi. The optical studies show that the QD film thickness required for efficient color conversion can be successfully realized even for the small pixel sizes. We further combine green and red pixels in a single converter to achieve white emission when combined with blue LED emission. The QD color converter design and processing are decoupled from the LED fabrication and can be easily scaled to wafer-size integration with arbitrary pixel sizes for QD-based RGB displays with ultrahigh resolution.

Strong and Localized Luminescence from Interface Bubbles Between Stacked hBN Multilayers.

Extraordinary optoelectronic properties of van der Waals (vdW) heterostructures can be tuned via strain caused by mechanical deformation. Here, we demonstrate strong and localized luminescence in the ultraviolet region from interface bubbles between stacked multilayers of hexagonal boron nitride (hBN). Compared to bubbles in stacked monolayers, bubbles formed by stacking vdW multilayers show distinct mechanical behavior. We use this behavior to elucidate radius- and thickness-dependent bubble geometry and the resulting strain across the bubble, from which we establish the thickness-dependent bending rigidity of hBN multilayers. We then utilize the polymeric material confined within the bubbles to modify the bubble geometry under electron beam irradiation, resulting in strong luminescence and formation of optical standing waves. Our results open a route to design and modulate microscopic-scale optical cavities via strain engineering in vdW materials, which we suggest will be relevant to both fundamental mechanical studies and optoelectronic applications.

Nanoscale Modification of WS2 Trion Emission by Its Local Electromagnetic Environment.

Structural, electronic, and chemical nanoscale modifications of transition metal dichalcogenide monolayers alter their optical properties. A key missing element for complete control is a direct spatial correlation of optical response to nanoscale modifications due to the large gap in spatial resolution between optical spectroscopy and nanometer-resolved techniques. Here, we bridge this gap by obtaining nanometer-resolved optical properties using electron spectroscopy at cryogenic temperatures, specifically electron energy loss spectroscopy for absorption and cathodoluminescence for emission, which are then directly correlated to chemical and structural information. In an h-BN/WS2/h-BN heterostructure, we observe local modulation of the trion (X-) emission due to tens of nanometer wide dielectric patches. Trion emission also increases in regions where charge accumulation occurs, close to the carbon film supporting the heterostructures. The localized exciton emission (L) detected here is not correlated to strain above 1%, suggesting point defects might be involved in their formation.

Tunable Optical Properties of Thin Films Controlled by the Interface Twist Angle.

Control of materials properties has been the driving force of modern technologies. So far, materials properties have been modulated by their composition, structure, and size. Here, by using cathodoluminescence in a scanning transmission electron microscope, we show that the optical properties of stacked, >100 nm thick hexagonal boron nitride (hBN) films can be continuously tuned by their relative twist angles. Due to the formation of a moiré superlattice between the two interface layers of the twisted films, a new moiré sub-band gap is formed with continuously decreasing magnitude as a function of the twist angle, resulting in tunable luminescence wavelength and intensity increase of >40×. Our results demonstrate that moiré phenomena extend beyond monolayer-based systems and can be preserved in a technologically relevant, bulklike material at room temperature, dominating optical properties of hBN films for applications in medicine, environmental, or information technologies.

Dry Transfer of van der Waals Crystals to Noble Metal Surfaces To Enable Characterization of Buried Interfaces.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have been explored for many optoelectronic applications. Most of these applications require them to be on insulating substrates. However, for many fundamental property characterizations, such as mapping surface potential or conductance, insulating substrates are nonideal as they lead to charging and doping effects or impose the inhomogeneity of their charge environment on the atomically thin 2D layers. Here, we report a simple method of residue-free dry transfer of 2D TMDC crystal layers. This method is enabled via noble-metal (gold, silver) thin films and allows comprehensive nanoscale characterization of transferred TMDC crystals with multiple scanning probe microscopy techniques. In particular, intimate contact with underlying metal allows efficient tip-enhanced Raman scattering characterization, providing high spatial resolution (<20 nm) for Raman spectroscopy. Further, scanning Kelvin probe force microscopy allows high-resolution mapping of surface potential on transferred crystals, revealing their spatially varying structural and electronic properties. The layer-dependent contact potential difference is clearly observed and explained by charge transfer from contacts with Au and Ag. The demonstrated sample preparation technique can be generalized to probe many different 2D material surfaces and has broad implications in understanding of the metal contacts and buried interfaces in 2D material-based devices.

Nanopores in 2D MoS2: Defect-Mediated Formation and Density Modulation.

Oxidation is a scalable process for introducing nanopores in two-dimensional transitional metal dichalcogenides (TMDs) for membrane applications. The nanopore density is determined by the areal density of their nucleation sites; understanding the nature of the defects and their control would enable tailoring of TMD membranes for targeted applications. In this work, we show that the nanopore distribution is dramatically different in strained and unstrained MoS2 crystals. We correlate this spatial distribution to the underlying arrangement of dislocations in MoS2 crystals, in contrast to previously suggested sulfur vacancies. To control the nucleation density of MoS2 nanopores, we demonstrate that the pore density can be modulated by electron beam exposure prior to the nanopore formation. Raman analysis of electron beam-exposed samples indicates that hydrocarbon adsorption activates defect species other than dislocations, which significantly enhances the nanopore density in MoS2.

Light Management in Organic Photovoltaics Processed in Ambient Conditions Using ZnO Nanowire and Antireflection Layer with Nanocone Array.

Low carrier mobility and lifetime in semiconductor polymers are some of the main challenges facing the field of organic photovoltaics (OPV) in the quest for efficient devices with high current density. Finding novel strategies such as device structure engineering is a key pathway toward addressing this issue. In this work, the light absorption and carrier collection of OPV devices are improved by employment of ZnO nanowire (NW) arrays with an optimum NW length (50 nm) and antireflection (AR) film with nanocone structure. The optical characterization results show that ZnO NW increases the transmittance of the electron transporting layer as well as the absorption of the polymer blend. Moreover, the as-deposited polymer blend on the ZnO NW array shows better charge transfer as compared to the planar sample. By employing PC70BM:PV2000 as a promising air-stable active-layer, power conversion efficiencies of 9.8% and 10.1% are achieved for NW devices without and with an AR film, indicating 22.5% and 26.2% enhancement in PCE as compared to that of planar device. Moreover, it is shown that the AR film enhances the water-repellent ability of the OPV device.

Crystallographic character of grain boundaries resistant to hydrogen-assisted fracture in Ni-base alloy 725.

Hydrogen embrittlement (HE) causes sudden, costly failures of metal components across a wide range of industries. Yet, despite over a century of research, the physical mechanisms of HE are too poorly understood to predict HE-induced failures with confidence. We use non-destructive, synchrotron-based techniques to investigate the relationship between the crystallographic character of grain boundaries and their susceptibility to hydrogen-assisted fracture in a nickel superalloy. Our data lead us to identify a class of grain boundaries with striking resistance to hydrogen-assisted crack propagation: boundaries with low-index planes (BLIPs). BLIPs are boundaries where at least one of the neighboring grains has a low Miller index facet-{001}, {011}, or {111}-along the grain boundary plane. These boundaries deflect propagating cracks, toughening the material and improving its HE resistance. Our finding paves the way to improved predictions of HE based on the density and distribution of BLIPs in metal microstructures.

Two-Dimensional MoxW1-xS2 Graded Alloys: Growth and Optical Properties.

Two-dimensional (2D) transition metal dichalcogenides can be alloyed by substitution at the metal atom site with negligible effect on lattice strain, but with significant influence on optical and electrical properties. In this work, we establish the relationship between composition and optical properties of the MoxW1-xS2 alloy by investigating the effect of continuously-varying composition on photoluminescence intensity. We developed a new process for growth of two-dimensional MoxW1-xS2 alloys that span nearly the full composition range along a single crystal, thus avoiding any sample-related heterogeneities. The graded alloy crystals were grown using a diffusion-based chemical vapor deposition (CVD) method that starts by synthesizing a WS2 crystal with a graded point defect distribution, followed by Mo alloying in the second stage. We show that point defects promote the diffusion and alloying, as confirmed by Raman and photoluminescence measurements, density functional theory calculations of the reaction path, and observation that no alloying occurs in CVD-treated exfoliated crystals with low defect density. We observe a significant dependence of the optical quantum yield as a function of the alloy composition reaching the maximum intensity for the equicompositional Mo0.5W0.5S2 alloy. Furthermore, we map the growth-induced strain distribution within the alloyed crystals to decouple composition and strain effects on optical properties: at the same composition, we observe significant decrease in quantum yield with induced strain. Our approach is generally applicable to other 2D materials as well as the optimization of other composition-dependent properties within a single crystal.

Minority Carrier Transport in Lead Sulfide Quantum Dot Photovoltaics.

Lead sulfide quantum dots (PbS QDs) are an attractive material system for the development of low-cost photovoltaics (PV) due to their ease of processing and stability in air, with certified power conversion efficiencies exceeding 11%. However, even the best PbS QD PV devices are limited by diffusive transport, as the optical absorption length exceeds the minority carrier diffusion length. Understanding minority carrier transport in these devices will therefore be critical for future efficiency improvement. We utilize cross-sectional electron beam-induced current (EBIC) microscopy and develop methodology to quantify minority carrier diffusion length in PbS QD PV devices. We show that holes are the minority carriers in tetrabutylammonium iodide (TBAI)-treated PbS QD films due to the formation of a p-n junction with an ethanedithiol (EDT)-treated QD layer, whereas a heterojunction with n-type ZnO forms a weaker n+-n junction. This indicates that modifying the standard device architecture to include a p-type window layer would further boost the performance of PbS QD PV devices. Furthermore, quantitative EBIC measurements yield a lower bound of 110 nm for the hole diffusion length in TBAI-treated PbS QD films, which informs design rules for planar and ordered bulk heterojunction PV devices. Finally, the low-energy EBIC approach developed in our work is generally applicable to other emerging thin-film PV absorber materials with nanoscale diffusion lengths.

Room Temperature Sensing Achieved by GaAs Nanowires and oCVD Polymer Coating.

Novel structures comprised of GaAs nanowire arrays conformally coated with conducting polymers (poly(3,4-ethylenedioxythiophene) (PEDOT) or poly(3,4-ethylenedioxythiophene-co-3-thiophene acetic acid) display both sensitivity and selectivity to a variety of volatile organic chemicals. A key feature is room temperature operation, so that neither a heater nor the power it would consume, is required. It is a distinct difference from traditional metal oxide sensors, which typically require elevated operational temperature. The GaAs nanowires are prepared directly via self-seeded metal-organic chemical deposition, and conducting polymers are deposited on GaAs nanowires using oxidative chemical vapor deposition (oCVD). The range of thickness for the oCVD layer is between 100 and 200 nm, which is controlled by changing the deposition time. X-ray diffraction analysis indicates an edge-on alignment of the crystalline structure of the PEDOT coating layer on GaAs nanowires. In addition, the positive correlation between the improvement of sensitivity and the increasing nanowire density is demonstrated. Furthermore, the effect of different oCVD coating materials is studied. The sensing mechanism is also discussed with studies considering both nanowire density and polymer types. Overall, the novel structure exhibits good sensitivity and selectivity in gas sensing, and provides a promising platform for future sensor design.

A Two-Step Absorber Deposition Approach To Overcome Shunt Losses in Thin-Film Solar Cells: Using Tin Sulfide as a Proof-of-Concept Material System.

As novel absorber materials are developed and screened for their photovoltaic (PV) properties, the challenge remains to reproducibly test promising candidates for high-performing PV devices. Many early-stage devices are prone to device shunting due to pinholes in the absorber layer, producing "false-negative" results. Here, we demonstrate a device engineering solution toward a robust device architecture, using a two-step absorber deposition approach. We use tin sulfide (SnS) as a test absorber material. The SnS bulk is processed at high temperature (400 °C) to stimulate grain growth, followed by a much thinner, low-temperature (200 °C) absorber deposition. At a lower process temperature, the thin absorber overlayer contains significantly smaller, densely packed grains, which are likely to provide a continuous coating and fill pinholes in the underlying absorber bulk. We compare this two-step approach to the more standard approach of using a semi-insulating buffer layer directly on top of the annealed absorber bulk, and we demonstrate a more than 3.5× superior shunt resistance Rsh with smaller standard error σRsh. Electron-beam-induced current (EBIC) measurements indicate a lower density of pinholes in the SnS absorber bulk when using the two-step absorber deposition approach. We correlate those findings to improvements in the device performance and device performance reproducibility.

Diffusion-Mediated Synthesis of MoS2/WS2 Lateral Heterostructures.

Controlled growth of two-dimensional transition metal dichalcogenide (TMD) lateral heterostructures would enable on-demand tuning of electronic and optoelectronic properties in this new class of materials. Prior to this work, compositional modulations in lateral TMD heterostructures have been considered to depend solely on the growth chronology. We show that in-plane diffusion can play a significant role in the chemical vapor deposition of MoS2/WS2 lateral heterostructures leading to a variety of nontrivial structures whose composition does not necessarily follow the growth order. Optical, structural, and compositional studies of TMD crystals captured at different growth temperatures and in different diffusion stages suggest that compositional mixing versus segregation are favored at high and low growth temperatures, respectively. The observed diffusion mechanism will expand the realm of possible lateral heterostructures, particularly ones that cannot be synthesized using traditional methods.

Oxidative Chemical Vapor Deposition of Neutral Hole Transporting Polymer for Enhanced Solar Cell Efficiency and Lifetime.

The concept of a neutral hole-transporting polymer is realized for the first time, by integrating patterned Cl(-) -doped poly(3,4-dimethoxythiophene) thin films into organic solar cells through a vacuum-based polymer vapor printing technique. Due to this novel polymer's neutrality, high transparency, good conductivity, and appropriate energy levels, the solar-cell efficiency and lifetime are significantly enhanced.

Transition Metal-Oxide Free Perovskite Solar Cells Enabled by a New Organic Charge Transport Layer.

Various electron and hole transport layers have been used to develop high-efficiency perovskite solar cells. To achieve low-temperature solution processing of perovskite solar cells, organic n-type materials are employed to replace the metal oxide electron transport layer (ETL). Although PCBM (phenyl-C61-butyric acid methyl ester) has been widely used for this application, its morphological instability in films (i.e., aggregation) is detrimental. Herein, we demonstrate the synthesis of a new fullerene derivative (isobenzofulvene-C60-epoxide, IBF-Ep) that serves as an electron transporting material for methylammonium mixed lead halide-based perovskite (CH3NH3PbI(3-x)Cl(x)) solar cells, both in the normal and inverted device configurations. We demonstrate that IBF-Ep has superior morphological stability compared to the conventional acceptor, PCBM. IBF-Ep provides higher photovoltaic device performance as compared to PCBM (6.9% vs 2.5% in the normal and 9.0% vs 5.3% in the inverted device configuration). Moreover, IBF-Ep devices show superior tolerance to high humidity (90%) in air. By reaching power conversion efficiencies up to 9.0% for the inverted devices with IBF-Ep as the ETL, we demonstrate the potential of this new material as an alternative to metal oxides for perovskite solar cells processed in air.

Nanoscale mapping of plasmon and exciton in ZnO tetrapods coupled with Au nanoparticles.

Metallic nanoparticles can be used to enhance optical absorption or emission in semiconductors, thanks to a strong interaction of collective excitations of free charges (plasmons) with electromagnetic fields. Herein we present direct imaging at the nanoscale of plasmon-exciton coupling in Au/ZnO nanostructures by combining scanning transmission electron energy loss and cathodoluminescence spectroscopy and mapping. The Au nanoparticles (~30 nm in diameter) are grown in-situ on ZnO nanotetrapods by means of a photochemical process without the need of binding agents or capping molecules, resulting in clean interfaces. Interestingly, the Au plasmon resonance is localized at the Au/vacuum interface, rather than presenting an isotropic distribution around the nanoparticle. On the contrary, a localization of the ZnO signal has been observed inside the Au nanoparticle, as also confirmed by numerical simulations.

Impacts of Ion Segregation on Local Optical Properties in Mixed Halide Perovskite Films.

Despite the recent astronomical success of organic-inorganic perovskite solar cells (PSCs), the impact of microscale film inhomogeneities on device performance remains poorly understood. In this work, we study CH3NH3PbI3 perovskite films using cathodoluminescence in scanning transmission electron microscopy and show that localized regions with increased cathodoluminescence intensity correspond to iodide-enriched regions. These observations constitute direct evidence that nanoscale stoichiometric variations produce corresponding inhomogeneities in film cathodoluminescence intensity. Moreover, we observe the emergence of high-energy transitions attributed to beam induced iodide segregation, which may mirror the effects of ion migration during PSC operation. Our results demonstrate that such ion segregation can fundamentally change the local optical and microstructural properties of organic-inorganic perovskite films in the course of normal device operation and therefore address the observed complex and unpredictable behavior in PSC devices.

Dimensional Tailoring of Hydrothermally Grown Zinc Oxide Nanowire Arrays.

Hydrothermally synthesized ZnO nanowire arrays are critical components in a range of nanostructured semiconductor devices. The device performance is governed by relevant nanowire morphological parameters that cannot be fully controlled during bulk hydrothermal synthesis due to its transient nature. Here, we maintain homeostatic zinc concentration, pH, and temperature by employing continuous flow synthesis and demonstrate independent tailoring of nanowire array dimensions including areal density, length, and diameter on device-relevant length scales. By applying diffusion/reaction-limited analysis, we separate the effect of local diffusive transport from the c-plane surface reaction rate and identify direct incorporation as the c-plane growth mechanism. Our analysis defines guidelines for precise and independent control of the nanowire length and diameter by operating in rate-limiting regimes. We validate its utility by using surface adsorbents that limit reaction rate to obtain spatially uniform vertical growth rates across a patterned substrate.