This PDF file contains the front matter associated with SPIE Proceedings Volume 11465, including the Title Page, Copyright information, and Table of Contents.

Welcome to the Low-Dimensional Materials and Devices 2020 conference

Raman scattering and photoluminescence measurements are useful to investigate spatially-resolved energy landscapes in two-dimensional layered structures. For example, charge transfer and electronic properties can be explored by optical phonons and excitons in various layered structures. Particularly, simultaneous Raman and photoluminescence imaging over a large area of two-dimensional surface can provide quantitative information on doping distribution. Further, in situ Raman imaging of operating field-effect transistor can be utilized to understand energy dissipation mechanism over an entire transport channel. [This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Grant No. 2019R1A2C1003366).]

We demonstrated a facile recipe using nano-optical and topographical measurements to estimate nanoscale strain in 2D materials. This method is generally applicable to nanostrained regions of arbitrary shape and can be easily applied to complex fields of interacting nanobubbles. Using state of the art nano-optical characterization tools, we show that the photoluminescence energy shift agrees with the estimated strain of single non-circular nanobubble. By combining the simultaneously solved strain maps of dozens of irregular nanobubbles, we reproduce the Grüneisen parameter of the in-plane Raman active mode for the 1L-WS2, showing the quantitative accuracy of our method for truly “nano” nanobubbles with radii on order 10-100 nm.

The simulation models of gallium nitride (GaN) nanostructure arrays are designed by COMSOL Multiphysics software package based on finite element method (FEM). The effects of changes of GaN nanowires in axial geometry on photoelectron concentration distributions performance are analyzed. And the absorptivity differences of nanorods, truncated-cones and inverted-pencil structures in ultraviolet band are studied. Analysis of simulation results pointed out that both truncated-corn arrays and inverted-pencil arrays have the ability of boosting light absorption compared to nanorod arrays. Thus, such positive conclusion of the axial structure changed GaN nanowires would offer new references for the photoemission layer design of the photocathode.

The p-n junction is a fundamental and ubiquitous component in modern electronic devices. It can be found in solar cells, light emitting diodes, and transistors. Equipped with chiral fermions, an electric field tunable band gap, and an accessible valley degree of freedom, Bernal stacked bilayer graphene (BLG) based p-n junctions are a promising platform for the development of new electronic devices. Here we report direct visualization of the electronic states inside and outside of circular BLG p-n junctions by using a scanning tunneling microscope. Unexpectedly, we discover that states within such junctions exhibit a robust C_3 symmetry. By using a numerical tight binding approach to model our p-n junctions we find that the observed states originate from the confinement of charges with gapped and trigonally warped bands. Our work indicates that new valley sensitive devices such as polarizers and splitters can be realized with BLG p-n junctions.

Introducing interfaces through heterostructuring is a common strategy to bring novel functionalities and improve device performances. Emerging nanomaterials, of which heterostructuring is under development, can be utilized with combinations of various conventional semiconductors, such as Si, Ge, and compound semiconductors. I’ll present recent achievements of fabrication of hybrid architectures composed of 2D and conventional materials. The experimental approach was epitaxial growth Si, Ge, and ZnO on various 2D materials including graphene, hexagonal boron nitride, transition metal dichalcogenides. Van der Waals and remote epitaxy techniques were employed. Details of nucleation control, charge transfer in hybrid architectures, and insight of device applications will be discussed.

Efficient doping of 2D materials, including carrier type, concentration and mobility, is challenging but essential for enabling their future electronic and photonic applications. We are developing substitutional n- and p- doping of InSe semiconductor by introducing Sn and Zn, respectively, in the Bridgman bulk crystal growth. Electrical transport properties of undoped vs. n- and p- doped InSe crystals are compared by conducting Hall measurements on bulk crystals and FET transport measurements on exfoliated thin layers. Undoped InSe is intrinsically n-type in both bulk and thin-film forms, with [n]~3.5E14 cm-3 and mu values of up to 1,400 cm2 V-1 s-1 for thick layers at 300K. Carrier concentration in Sn-doped thick layers increases approximately two-fold, while the corresponding mobility reduces ~2 times at 300 K. Zn-doped InSe shows p- behavior for bulk InSe with [p]~7.9E13 cm-3 and mu~43 cm2 V-1 s-1 at 300 K, which reverts to ambipolar/n- type behavior for thin layers in FET devices.

2D materials have a number of intriguing value proposition that could be harnessed for compact, tunable, high-performance optoelectronic devices when heterogeneously integrated in photonic circuits. Here I review our latest work including; (1) tunable TMD-based microring resonator with engineered critical-coupling condition, (2) a broadband graphene plasmon-slot detector (R=0.7A/W), (3) a bandgap-shifted strain-engineered absorption-enhanced MoTe2 photodetector at 1.55um (R=0.5A/W, low-dark-current <10nA@-1V), (4) a record-high responsivity (R=1.4A/W) slot-plasmon exciton-modulated MoTe2 detector, (5) a MoS2 electro-absorption modulator all enabled by our recently developed method of cross-contamination-free yet deterministic dry transfer 2D material ‘printer’ mimicking a 3D printer for enabling rapid prototyping. These devices are based on heterogeneous integration of 2D materials into Silicon and SiN photonics, with the latter used for on-exciton modulation or exciton absorption.

Metal-semiconductor contacts have been the subject of intense investigation and study for several decades. With the advent of atomically-thin semiconductors new opportunities have emerged in investigating these mysterious yet, critical buried interfaces. In this talk we will show our recent advances in probing the nature of these contacts by a variety of scanning probe techniques. The primary model system will be that a bulk noble metal such as gold or silver and a 2D chalcogenides semiconductor such a molybdenum disulfide (MoS2). We will discuss impact of contact type and evaporation technique on the electrical and optical properties of the junction and suggest ways to make idealized contacts. We will then extend our analysis to looking at the same interface from an optical perspective and investigating hybrid states of excitons and plasmons observed via near-field photoluminescence micro-spectroscopy.

Hybrid graphene photodetectors were prepared with endohedral fullerenes deposited on graphene using electrophoretic methods. Endohedral Sc3N@C80, which acts as an electron acceptor was used, and the ensuing electronic and optoelectronic properties were measured. Another endohedral fullerene, La@C82 adsorbed on graphene acted as an electron donor. Upon optical illumination, for the Sc₃N@C₈₀-graphene hybrid, the photoinduced free holes are injected into graphene, increasing the hole carrier concentration in graphene, while the photoexcited electrons remain in Sc₃N@C₈₀. We will discuss the photodetector response of these 0D-2D systems with both Sc₃N@C₈₀-graphene and La@C82-graphene hybrids. The exceptional performance gains achieved with both types of hybrid structures confirms the potential of endohedrals to dope graphene for high performance optoelectronic devices using a facile and scalable fabrication process.

Miniaturization of electronic devices and progress in surface science demand novel, powerful microscopy methods for material characterization on a length scale of only a few atomic distances. This paper discusses application of the combined x-ray photoelectron microscopy / low-energy electron microscopy (XPEEM/LEEM) system to studying of the structural, electronic and chemical properties of surfaces at nanometer scale. Several examples are given, focusing on the comprehensive spectro-microscopic investigations of 2D structures, including epitaxially grown films as well as exfoliated, μm-size thin flakes of 2D van der Waals materials. Benefitting from the high brilliance of the synchrotron, and utilizing its capabilities for in-situ sample preparation and treatment, the XPEEM/LEEM is a powerful tool for comprehensive characterization of static and dynamic properties of surfaces and interfaces, and it is particularly suited for comprehensive investigation of 2D materials, down to single monolayers.

In the emerging field of twistronics, new electronic devices based on bilayer graphene have shown distinct electronic properties that depend on the rotational misalignment of one crystalline layer with respect to another. Given present methods of preparing these bilayers, there is always some uncertainty in the actual versus targeted twist angle of a specific bilayer that can only be resolved by measuring the moiré patterns that are unique to a specific twist angle. Traditional methods enabling such a measurement, Transmission Electron Microscopy and Scanning Tunneling Microscopy, impose serious restrictions on the types of substrates supporting the bilayers, which, in turn, constrains the subsequent fabrication of any devices. We report here a new, non-destructive method to measure moiré patterns of bilayer graphene deposited on any smooth substrate, using the scanning probe technique known as scanning microwave impedance microscopy (sMIM) which enables the simultaneous generation of localized topography, capacitance and conductance images with nanometer 1 scale resolution . Moiré patterns were observed in samples prepared on various substrates with twist angles ranging from 0.02 to 6.7 degrees, beyond which the moiré patterns are too small to be resolved by the sMIM probes. We present some possible reasons for the various contrast mechanisms. Addressing the problem of variations across a bilayer surface due to localized moiré distortions that result from the tensile and shear forces involved in transferring a twisted bilayer to a substrate, we demonstrate how sMIM can precisely map the twist angle distribution across the film, and enable direct device and circuit routing.

Here we present advances in the use of near field microwaves, in both high-resolution microwave microscopy of nanostructured materials, as well as on the processing of materials with high microwave power density in selected areas. For the former, we utilize a commercial Scanning Microwave Impedance Microscope (PrimeNano Inc.) and demonstrate 3nm feature size. We discuss the mechanisms for image contrast, as well as draw equivalent circuits for a variety of metallic, semiconductor and dielectric systems. For the latter, we built a homemade system that enabled the concentration of microwaves in areas smaller than 100um diameter. We report on the system performance, and demonstrate the use of the tool in monitoring materials changes as heating takes place, by measuring and analyzing the reflected microwave signal. In summary, we demonstrate the usage, limits and opportunities of harnessing microwave power in the near field to map and modify materials properties in small scales.

We fabricated hybrid nanostructures consisting of MoS2 monolayers and plasmonic Au nanogratings. The polarization dependence of optical reflectivity showed a clear feature of surface plasmon polariton excitation. The MoS2 monolayers on Au nanogratings exhibited significantly enhanced photoluminescence intensity, compared with those on flat SiO2/Si substrates and Au thin films. Polarization-dependent surface photovoltage mapping was also obtained by Kelvin-probe force microscopy to study the exciton-plasmon coupling in the samples. In this presentation, we will discuss the interplay among photons, excitons, and surface plasmons in the MoS2-metal nanostructures.

Lead iodide (PbI2) is a layered material with unique optical and electrical properties. Despite being extensively studied in its bulk form and being used as a precursor for perovskite materials, the recently developed PbI2 nanosheets have shown a great promise for high-performance optoelectronic devices, such as flexible photodetectors, heterostructure photodetectors and nanolasers. These novel applications of PbI2 nanosheets, however, require careful characterisation of their crystalline structure. Here we experimentally and numerically demonstrate the nonlinear microscopy of PbI2 nanosheets. It is a non-invasive optical technique to precisely determine the thickness, crystalline orientation and strain.

Solid-state, vacuum nanoelectronic devices have the potential to combine the advantages of vacuum electron devices, such as robustness in harsh environments and high frequency operation, and solid-state devices, such as size, integrability, and low-power operation. In this work, we demonstrate novel GaN nanogap field emission diodes that operate in air and exhibit low turn-on voltage, high field emission current, and excellent on-off ratio. We present experimental and modeling results on the field emission characteristics of these devices at various nanogap sizes and operating pressures. These results provide critical new insights into the behavior of this new class of devices and point to future challenges and opportunities. Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

We introduce a novel fin LED architecture that eliminates “efficiency droop”. Efficiency droop, which is the decline in internal quantum efficiency with increasing current density, is one of the significant challenges facing wide bandgap LED. In this presentation, we present data on how nanofins as quasi one-dimensional nanoLEDs would operate at low and high injection current densities and how electron-hole recombination is influenced because of the shape and size of the recombination region. Results include temperature-dependent electroluminescence of fin LEDs as well as dependence of their total radiant flux (optical output power) on injected current. We show unique examples of droop-free operation of fin LEDs at extremely high injection current densities beyond 1 million A/cm2. A three-parameter model based on the radiative and nonradiative processes (ABC) including the SRH and Auger recombination is presented that provides insight on how the non-radiative Auger recombination proces

White light quantum dots (WQDs) such as CdSe and ZnxCd1-xS are considered source candidates in solid-state lighting (SSL) due to their unique optical properties. However, the low fluorescence quantum yield (QY, < 60 %), low luminous efficacy (~11.9 lm/W), and poor stability (~100 hr) limits their commercial application. In order to solve the above problems, a high QY of ZnxCd_1-xS WQDs (x=0.2, 0.5 and 0.8, named as Zn0.2, Zn0.5 and Zn0.8, respectively) was prepared by using unsaturated fatty acid-oleic acid (OA) and non-coordinating solvent-1-octadecene (ODE) at low temperature (180 °C). Moreover, the Zn0.5 WQDs was coated with silica to improve stability, and named as Zn0.5@SiO₂. The results show that the QYs of Zn_0.2, Zn_0.5 and Zn_0.8 WQDs are 82, 86 and 51 %, and the TEM images show that the morphologies of those WQDs are spherical, and the particle size are 2.7, 2.6 and 2.5 nm, respectively. The particle size of Zn_0.5 WQDs in Zn_0.5@SiO₂ is 2.6 nm, and the emission wavelength is red-shifted from 391/501 to 418/533 nm due to agglomeration. The CIE coordinates, color rendering index (CRI), correlated color temperature (CCT) and luminous efficacy of Zn0.5-based white light-emitting diode (WLED) are (0.36, 0.43), 73, 4784 K and 30.6 lm/W, respectively. The CIE coordinates, CRI, CCT and luminous efficacy of Zn0.5@SiO2-based WLED are (0.42, 0.46), 72, 3732 K and 19.4 lm/W, respectively. After long-term operation for 261 h, the luminous efficacy of Zn_0.5- and Zn_0.5@SiO₂-based WLED decreased by 82 and 14 %, respectively. The above results confirm that ZnxCd_1-xS WQDs with high QY can be successfully prepared at low temperature by using unsaturated fatty and non-coordinating solvent, and coating the WQDs with silica can increase WLED stability. This result can make WQD a big step towards the application of solid state lighting.

Devices with negative transconductance (NTC) or negative differential resistance (NDR) have been attracting a great deal of attention in novel device applications because of their unique electrical characteristics. In such devices, there exist operation regions in which current decreases with an increase of bias voltage, whereas current monotonically increases with an increase of bias voltage in conventional devices. The NTC devices enable realization of multi-valued logic circuits, where higher bit density and lower power dissipation can be achieved by reduction of the number of devices and interconnects in integrated circuits. In this work, gate-tunable diodes with NTC will be demonstrated by constructing p-n heterojunctions composed of inkjet printed single-walled carbon nanotubes (SWCNTs) and indium oxide (InO). Surface conditions for the inkjet printed layers are adequately modified by plasma treatment to form continuous semiconducting layers on both dielectric and underlying semiconducting layers for the formation of partially overlapped p-n heterojunctions. The resultant devices show anti-ambipolar behavior, where drain-current increases until it reaches the maximum value then decreases as gate-voltage increases, which is opposite to the ambipolar behavior. The device characteristics based on partially overlapped p-n heterojunction and fully overlapped bilayer p-n heterojunction will be compared. Additional thirty printed heterojunction-based devices are fabricated, and the device characteristics are statistically analyzed to show their potential for reliable and scalable applications. Finally, inkjet printed ternary inverters, which possess three logic states, will be demonstrated by employing inkjet printed gate-tunable diodes with SWCNT transistors.

Neuromorphic computers based on analogue neural networks aim to lower computing power by reducing the need to shuttle data between memory and logic units. Artificial synapses containing nonvolatile analogue conductance states enable direct computation using memory elements; however, most nonvolatile analogue memories require high write voltages and large current densities and are accompanied by nonlinear and unpredictable weight updates. We develop an inorganic redox transistor based on electrochemical lithium-ion insertion into LiXTiO2 that displays linear weight updates at both low current densities and low write voltages. The write voltage, as low as 200 mV at room temperature, is achieved by minimizing the open-circuit voltage and using a low-voltage diffusive memristor selector. We further show that the LiXTiO2 redox transistor can achieve an extremely sharp transistor subthreshold slope of just 40 mV/decade when operating in an electrochemically driven phase transformation.

Processing methods employing laser are often used to tailor semiconductors. For example, crystallization of semiconductors is induced via the interaction between laser and semiconductors – laser crystallization. While laser crystallization is widely used in preparing thin films made of a single chemical element, extending its use for those containing multiple chemical elements (e.g., metal oxides) could unfold applications currently not achievable. In this paper, a continuous-wave laser diode with a micrometer-scale chevron-shaped beam profile – micro chevron laser beam (μ-CLB) – is exploited to crystallize non-single-crystal CuO thin films deposited on fused silica substrates. Resulting single-crystal Cu2O strips are studied to reveal their unique characteristics associated with key parameters of the crystallization process.

Depositing thin films is often limited to a specific deposition process by which precursors are transported and reacted in a deposition environment. In other words, a deposition environment in which two deposition processes are unified should offer a new perception of devising a thin film structure, which galvanizes our combining atomic layer deposition (ALD) and magnetron sputtering (SPU) in a single chamber – sputtering atomic layer augmented deposition (SALAD). The SALAD system offers advantages of both ALD capable of delivering precursor precisely and accurately and SPU versatile in choosing chemical elements. In this paper, the SALAD system is employed to deposit nanocomposites consisting of multiple pairs of an aluminum oxide thin film deposited by ALD and a copper thin film deposited by SPU. Optical properties collected from the nanocomposites show distinctive dispersion features to which the effective medium approximation does not seem to simply apply.

Biocompatibility of medical implants is a long-sought goal of the physical sciences and medical research communities. This concept refers to a material’s capacity to induce a response adequate to a bone host. However, identifying materials which are fulfilling this requirement for a wide range of different applications is still an unresolved issue. In this paper, are analyzed the properties of TiMo₂₀Si_xZr₇Ta₁₅ alloys with different ratios of Si (x=0; 0.5; 0.75;1.0) with possibilities of use for medical applications, using a set of complementary testing methods based on X-ray diffraction, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and resonant ultrasound spectroscopy (RUS). Across the compositions, the methods emphasize the mechanical properties of the sample, their microstructural characteristics, hardness and mechano-elastic properties, in order to establish a correlation between the structural parameters and mechanical properties of the sample, which are crucial in the understanding the feasibility of incorporating these alloys into prosthetics.

We fabricated stand-alone trilayers (3L) and their heterostructures of MoS2 and WS2 on Al2O3 substrates. In particular, two kinds of heterostructures with different stacking sequences (i.e., 3L-MoS2/3L-WS2 and 3L-WS2/3L-MoS2) were prepared for comparison. The light-induced surface potential change suggested that the internal electric field along the thickness direction was present in the stand-alone (MoS2 and WS2) and heterostructure samples. Relative peak shifts of the Raman spectra supported the presence of the internal field in our samples. Physical origins to induce the internal electric field will be discussed in the presentation.

Single-walled, armchair type carbon nanotube with mixed spins was investigated within the framework of Ising-like ferromagnetic model, by means of Monte Carlo technique based on the metropolis algorithm. According to this approach the influence of the size effect on the nanosystem behavior was investigated. The critical point of the system was obtained. The influence of the external magnetic field on the system parameters was studied. The interaction constant between nearest neighbor nodes and the temperature are the two major factors that influence the type of phase transitions in the considered nanosystem. The perspective of possible application of such carbon nanotubes in spintronics was shown.

CuInS₂/ZnS quantum dots (CIS/ZnS QDs) have wide full width at half maximum (FWHM) due to the emission mechanism caused by point defect. This property make CIS/ZnS QD suitable for application in solid-state lighting (SSL). However, it was mentioned in the literature that white light-emitting diode (WLED) using CIS/ZnS QDs as the conversion material has low color rendering (CRI is less than 70), and one of the methods to increase the color rendering is increasing the range of the emission spectrum. CIS QDs belong to the point defects emission mechanism, and non-stoichiometric chemical substitution can create a large number of point defects. Therefore, in this study, CIS/ZnS QDs were first prepared by traditional thermal injection method, and (ZnCuIn)S₂ (ZCIS) QDs were prepared by adding Zn²⁺, Cu⁺ and In³⁺ ions at the same time. The results show that the emission wavelengths, relative quantum yield (QY), FWHM and surface morphology of CIS/ZnS QDs are 535 nm, 83 % and 72 nm and tetrahedron, respectively. On the other hand, those are 540 nm, 32 %, 118 nm and spherical shape for ZCIS QDs. Moreover, the diffraction peaks are between chalcopyrite and zinc blende structure meaning that the samples have alloyed structure. The CIE coordinates and color rendering index (CRI) of CIS/ZnS-based WLED are (0.35, 0.31) and 68. The CRI can be improved from 68 to 90 for ZCIS-based WLED. From the above results, it is known that ZCIS QDs with a FWHM up to 118 nm can be prepared by non-stoichiometric chemical substitution diffusion method. The wide FWHM shows positive effect to increase CRI value of ZCIS-based WLED and this advantage helps ZCIS QDs to be more widely used in solid-state lighting applications.

In recent years, blue phosphorene (BP) becomes one of the emerging two-dimensional material due to the theoretical prediction of its thermal stability. In this communication, a comparative study of structural, photophysical and electronic properties of monolayer MoS₂/Blue Phosphorene and monolayer WS₂/Blue Phosphorene heterostructures has been done. Calculated Results show the interlayer distance of 3.15 Å and 3.23 Å for monolayer MoS₂/Blue Phosphorene and monolayer WS₂/Blue Phosphorene heterostructures respectively. These results indicate the presence of strong van der Waals coupling in the optimized heterostructures. The electronic behavior of these heterostructures has been studied by computing electronic band structure and density of states. A bandgap of 1.08 eV for a lattice mismatch of 2.4% in case of monolayer MoS₂/Blue Phosphorene heterostructure has been obtained while for the case of monolayer WS₂/Blue Phosphorene heterostructure the calculated bandgap is 1.41 eV for 2.6% lattice mismatch. Photophysical properties show a very high optical absorption coefficient in both the visible and ultraviolet spectrum. Type I and Type II band alignment have been formed for the monolayer MoS₂/Blue Phosphorene and monolayer WS₂/Blue Phosphorene heterostructures respectively. The existence of these fascinating properties suggests their potential utility as photocatalytic materials, and in the area of photovoltaic and optoelectronics.

We fabricated MoS2-Au hybrid nanostructures using nanosphere lithography and investigated their photoluminescence (PL) characteristics. Arrays of Au nanotriangles (NTs) and nanoholes (NHs) were fabricated for comparison. MoS2 monolayers on both NT and NH arrays exhibited enhanced PL intensity, compared with those on SiO2/Si substrates and flat Au thin films. Numerical simulations revealed clear distinction in the electric field intensity distributions in the NT and NH arrays at the PL excitation wavelength. Such difference could be attributed to the excitation of localized and propagating surface plasmon in the NT and NH arrays. This work helps us to understand how the plasmonic NT and NH arrays affect the physical properties of the MoS2 monolayers on them.

Two-dimensional semiconductors such as monolayer transition metal dichalcogenides (TMDs) exhibits remarkable optical properties such as robust valley polarization, making them ideal for optoelectronic and valleytronic devices. Manipulating the valley polarization by optical method is the key to realize valleytronic devices. Here, we demonstrate a resonant plasmonic nanostructure designed to spatially separate the emissions from different valleys of the WSe2 monolayer at cryogenic temperature. By changing the helicity of excitation, we show the directionality control of valley-based emission. Our hybrid nanostructure exhibits the possibility to realise the valleytronic devices.

This study presents a novel approach to enhance the photoluminescence and minimize strain in selfassembled bilayer InAs quantum dot (QDs). To obtain this approach, multi-level digital alloy capping layer (DACL) is introduced in the growth of the ternary InGaAs capping layer having different material composition, instead of conventional thick layer. A single thick 4 nm InGaAs capping on the InAs QD layers (Sample A1) is divided into four equal parts each having 1 nm thicknesses. The composition of indium (In) in strain reducing layer (SRL) in growth direction is varied from 45% to 15% for both first layer and second layer QDs (Sample D1). The experimental low temperature ground state emission wavelength for the sample A1 and sample D1 was 1096 and 1167 nm respectively. The biaxial and hydrostatic strain in growth direction was theoretically computed and compared for both analog and digital samples. The computed hydrostatic and biaxial strain in sample D1 is improved by 7.19% and 6.79% respectively, when compared to that of analog sample A1. The improved hydrostatic strain provides the better carrier confinement. The improved biaxial strain offers more band splitting between heavy-hole and light-hole band in valence band. This decreases the ground state band gap and thus offers a red shift in photoluminescence (PL). The experimental PL for both samples were also validated by simulating both heterostructures. The sample with DACL growth mode provided better crystalline quality, enhanced quantum yield and lesser defects.

We present a new concept of strain minimization through digital alloy capping layer (DACL) for the dot-in-a-well (DWELL) heterostructure. In this work, three different DWELL heterostructures with ternary capping of In_xGa_1-xAs have been simulated. The two analog DWELL structures with the indium (In) composition of 15% (Sample A1) and 45% (Sample A2), and one digital DWELL structure (Sample D1) with varying In composition from 45% to 15% in steps of 10 are considered. The bottom and top InGaAs well layer thickness of QD is considered to be 8 nm each. In digital sample D1, the bottom and top well layer is equally divided into 4 sub-parts, each having the thickness of 2 nm. The biaxial strain in sample D1 is increased by 0.66% and reduced by 2.95% and hydrostatic strain in sample D1 is reduced by 5.14% and increased by 3.49% when compare to their analog counterparts A1 and A2 respectively. Higher biaxial strain indicates the increased spacing between the heavy-hole and light-hole band in valence band, which would result in red-shifted photoluminescence. Lower hydrostatic strain ensures deeper electrostatic potential facilitating higher activation energy. The sample D1 provides the gradual strain relaxation inside the top and bottom InGaAs well layer. The emission wavelength of 1394, 1547, and 1514 nm is observed for samples A1, A2 and D1 respectively. The overall strain distribution inside the heterostructure is smoothly minimized in steps which would result in defect free heterostructure and hence will provide longer emission wavelength for SWIR applications.

Over the last decade InAs Stranski–Krastanov (S-K) quantum dots (QDs) grown on GaAs substrate have been widely explored for optoelectronic devices. In the recent past, the uncapped surface QDs are getting much attention for sensing application with adequate sensitivity to lower molar concentration of contaminants. Though non-uniform size distribution is an inherent property of the self-assembled S-K growth process, sensitivity of the surface QDs is significantly affected by this. Here, we have grown the surface QDs upon buried QD layer with a low barrier layer thickness to reduce the non-uniformity. In general, the 2D to 3D transition of the InAs QDs occurs only above 1.7 monolayer (ML). However, the 3D transition may be possible even at a lower monolayer coverage with the residual strain induced from the underlying QDs. In this study, particularly we have grown InAs surface QD layer at 1.6 ML coverage above the 2.7 ML buried QD layer with 8 nm thick GaAs spacer. The impact of vertically induced strain of the underneath InAs QD layer on the growth of surface dots has been investigated. The morphology of surface QDs is observed through Atomic force microscopy (AFM), which indicates the formation of uniform QDs with lower defects. The low temperature photoluminescence (PL) spectroscopy provides the evidence of the wave function overlap between the buried and surface QDs.

For years, colloidal quantum dots (cQDs) have been optimized to offer great optical properties customizable through varying size and composition, and become the mature nanomaterial they currently are. However, their optical properties are affected by their environment. Hence, optical behaviour inconsistencies across experiments emerge from using cQDs inside different matrices. Therefore, their scope of applications has hitherto been greatly limited to specific applications. Recently, a new protocol to incorporate cQDs inside step index polymer optical fibers was developed by Whittaker and co-workers. It enables a cheap and robust fabrication of light-generating optical fibers. In addition to shielding cQDs from their outer environment, optical fibers also support optical modes. Hence, according to cavity quantum electrodynamics principles, these modes must alter the local density of optical states of the system, thus modifying the decay rate for each emitter located within the optical fibers. The current work revealed through finite difference time domain simulations only minimal modifications of the decay rate of a single quantum emitter located at the center of a polystyrene/PMMA core/clad step index fiber. Purcell factors staying within 2.2% of its initial value upon changes of the core radius were obtained. Thus, the Purcell enhancement offered by step index fibers is negligible compared to that of current semiconductor microcavities. Consequently, more exotic fiber geometries that offer greater Purcell effect must be identified before fast and cheap light-generating optical fiber can be made from this fabrication process.

The strain-coupled InAs Stranski-Krastanov (SK) and submonolayer (SML) quantum dot (QD) heterostructure improves the optoelectronic properties of the infrared photodetector as compared to the single SK or SML heterostructures. The discussed heterostructures for SK on SML are grown with three different spacer layer thicknesses (5, 7.5 and 10 nm) between SK and SML dots and each sample is grown with two different growth rates (0.1 ML/sec and 0.05 ML/sec). In this study, we are modulating the optical and structural properties of the heterogeneously coupled SK on SML structure with ex-situ annealing treatment. Rapid thermal annealing at 650 ℃ and 700 ℃ is done for all samples. Photoluminescence (PL) result shows blue shift in the peak position for annealed samples compared to the as-grown samples. The reason for this blue shift can be (1) In out-diffusion from the dots, or (2) Diminution in dot size. A reduction in full width half maxima (FWHM) of the PL spectra along with the annealing temperature is observed, which confirms the smaller size of dots after annealing. Additionally, substantial variation in strain profile for these hybrid structures has been discerned by high resolution X-ray diffraction (HRXRD). Hence, this study would definitely help to optimize the ex-situ annealing temperature on heterogeneously coupled SK-SML structure for wavelength tunable photodetection in the near infrared regime.

Zinc oxide is a well-known wide bandgap semiconductor material. High exciton energy at room temperature makes it useful in optoelectronic applications. ZnO can be made available in bulk, film and nanostructured form as well. Nanostructures of ZnO have been proven to have augmented optical properties and extensively being used in optoelectronics and sensing applications. Here we have shown the effect of seed layer thickness on the chemical growth of ZnO nanorods. Two samples A and B has been prepared by hydrothermal bath synthesis having seed layer thicknesses of 120 nm and 350 nm, respectively. FEG-SEM images of nanorods revealed longer and high aspect ratio for sample B while sample A showed lower aspect ratio nanorods. XRD results indicate a dominant c-axis oriented (002) plane at around 33.5°. The calculated grain size of Sample A and B were 31.5 nm and 34.4 nm, respectively. Calculated strain values show a compressive strain in the crystal lattice. The Band edge PL peak emission for sample A and B was found to be at 368.74 nm and 368.38 nm at 18 K.

ZnMgO is extensively being used in UV based optoelectronic applications owing to its high bandgap and excitonic emissions at room temperature. In many applications, UV-Vis detection is required such as spectroscopy, medical applications and plasmonics. ZnMgO film and silicon can work in parallel to give a broad UV-Vis response. We fabricated ZnMgO/Si UV-Vis photo detector. UV-Ozone annealing can be used to increase the responsivity due to an increase in photoconductive gain. Here in this report, we have shown the effect of UV-Ozone on the transient behavior of the photodetector. The calculated responsivity values for 300 nm, 400 nm and white light were 3 A/W, 95 A/W and 50 A/W respectively. After UV-Ozone corresponding responsivity increased to 5 A/W, 180 A/W and 105 A/W respectively. The transient response of the photodetector was measured for asfabricated and UV-Ozone annealing. The transient behavior was fitted into two time constant of rise and fall time and switching parameters at 300 nm, 400 nm and white light are compared. The lesser time constant indicates carrier generation and recombination and the larger time constant is related with traps capture and release process. Rise time for 300 nm, 400 nm and white light was estimated to be 100, 10 and 8 ms respectively and the corresponding fall times were 140, 45 and 40 ms. Rise time for 300 nm illumination decreased from 100 to 70 ms after UV-Ozone.

This paper presents a novel heterostructure with multilayer Stranski-Krastanov (SK) quantum dots (QDs) heterogeneously coupled to Submonolayer (SML) QDs that shows better performance in terms of both optical and material characteristics. The 2.5 ML InAs/GaAs multilayer SK QDs are grown on the 6 stack 0.3 ML InAs SML QDs using a Molecular Beam Epitaxy system. Multilayer includes single, bi, tri, penta, hepta and ten-layer SK QDs and a different growth mechanism is adapted to maintain the dot size similar in each layer irrespective of the residual strain from the bottom layers. Photoluminescence (PL) and high resolution X-ray Diffraction (HRXRD) experiments are done in order to analyze the optical and material characteristics of these grown heterostructures. PL results show that the ground state peak wavelength of SK QD for all the heterostructures is at ~1035 nm, which confirms a uniform dot size for all heterostructures. However, the sample with ten SK QD layers coupled to SML QD has the highest luminescence intensity, lowest full width half maxima (FWHM: ~50 meV), and highest activation energy (~397 meV). A peak at ~947 nm in the PL spectra confirms the presence of SML QDs and the tunneling of carriers from ground state of SML QD to excited state of SK QD is observed. HRXRD results show that the compressive strain experienced by the QDs is reducing with increase in the number of SK QD layers, which depicts ten SK QD layers coupled to SML QD heterostructure as the best even in terms of material characteristics.

This study is focused on structural and optical properties of multilayer InAs/InGaAs dot-in-a-well (DWELL) heterostructure with varying capping layer thickness. Two samples A and B are considered with 2 monolayer (ML) InAs quantum dots (QDs). The top InGaAs capping layer thickness is varied from 6 to 8 nm from sample A to B, whereas the lower well thickness was kept constant at 2 nm. The ground-state peak in the photoluminescence (PL) spectra shows a blue shift with increased capping layer thickness (sample A: 1148 nm, sample B: 1140 nm). This blue shift is due to the increased well layer thickness, which leads to higher strain and shrinkage in dot size. The activation energy (Ea) is calculated from the temperature dependent PL results using the Arrhenius equation. The activation energy of sample A and B are 181 meV and 152 meV respectively. The higher activation energy leads to a reduction in dark current, which affirms that sample A would be better for the device application. Raman spectroscopy is also carried out to observe different phonon peaks in both samples. For sample B, the broadening of the GaAs transverse optic (TO) mode is higher, which may lead to increased non-radiative carrier recombination. It may also lead to reduction of carrier lifetime. Hence, sample A with lower well thickness would be useful in optoelectronic device application because of its improved optical characteristics.

Two dimensional (2-D) atomic layers have several advantages such as flexibility in choosing substrates, easy fabrication, large package area, minimum change of components, enhanced expiration date, and reduced defects by oxygen blocking structure for smart sensor. Individual graphene quantum dots (GQDs) become sensitive to H2 gas when their surfaces are decorated with Pd metal, and previous reports measure typical chemoresistive increases to be approximately 2-fold. Here, thousand-fold increases in resistance are demonstrated in the specific case where a Pd cluster decorates a GQDs defect site. Measurements on GQDs, performed both before and after defect incorporation, prove that defects have extraordinary consequences on the chemoresistive response, especially in the case of GQDs with metallic band structure.

Lead sulfide (PbS) quantum dots (QDs) exhibiting the narrow bandgap (Eg < 0.9 eV) provide a promising avenue to high performance, inexpensive shortwave infrared (SWIR) photodetector. However, most PbS-QD based SWIR photodiodes suffers from low responsivity and low external quantum efficiency (EQE) in the IR range due to insufficient ligand exchange. Here, we report a precursor engineering strategy that facilitates the commonly used Tetrabutylammonium iodide (TBAI) ligand exchange. We synthesized the PbS QDs from Lead Oxide (PbO) and lead acetate trihydrate (PbAc₂·₃H₂O). Compared with PbS QDs photodiode based on PbO, the responsivity and EQE of PbS QDs photodiode based on  PbAc2·3H2O have been improved from 0.15 to 0.586A/W and 12.35 to 47.27%.

Monolayers of transition metal dichalcogenides (TMD) are direct-gap semiconductor and exhibit strong roomtemperature photoluminescence (PL) in the visible and near-infrared range. The nature of the emission is commonly attributed to the formation of exciton states at room temperature. Monolayers of TMD with large surface area can nowadays be grown by chemical vapor deposition (CVD) on arbitrary substrates. However, spatial nonuniformities in the PL emission intensity and wavelength are commonly observed. Here, we combine high-resolution confocal optical microscopy with PL and Raman spectroscopy on CVD-grown WS2 monolayers. We observe an increased and a red-shifted PL emission from the center towards the edge of the individual grains. This red-shifted emission is due to the formation of charged and defect-bound excitons. Our multi-imaging analysis, by correlating PL and Raman emission, allow us to identify presence of local defects, low degree of crystallinity and oxidation.