Improved infrared photoluminescence characteristics from circularly ordered self-assembled Ge islands

The formation of circularly ordered Ge-islands on Si(001) has been achieved because of nonuniform strain field around the periphery of the holes patterned by focused ion beam in combination with a self-assembled growth using molecular beam epitaxy. The photoluminescence (PL) spectra obtained from patterned areas (i.e., ordered islands) show a significant signal enhancement, which sustained till 200 K, without any vertical stacking of islands. The origin of two activation energies in temperature-dependent PL spectra of the ordered islands has been explained in detail

Inter-Landau-level Andreev Reflection at the Dirac Point in a Graphene Quantum Hall State Coupled to a NbSe2 Superconductor

Superconductivity and quantum Hall effect are distinct states of matter occurring in apparently incompatible physical conditions. Recent theoretical developments suggest that the coupling of quantum Hall effect with a superconductor can provide a fertile ground for realizing exotic topological excitations such as non-abelian Majorana fermions or Fibonacci particles. As a step toward that goal, we report observation of Andreev reflection at the junction of a quantum Hall edge state in a single layer graphene and a quasi-two dimensional niobium diselenide (NbSe2) superconductor. Our principal finding is the observation of an anomalous finite-temperature conductance peak located precisely at the Dirac point, providing a definitive evidence for inter-Landau level Andreev reflection in a quantum Hall system. Our observations are well supported by detailed numerical simulations, which offer additional insight into the role of the edge states in Andreev physics. This study paves the way for investigating analogous Andreev reflection in a fractional quantum Hall system coupled to a superconductor to realize exotic quasiparticles.Comment: published verio

Spectroscopic comprehension of Mott-Hubbard insulator to negative charge transfer metal transition in LaNi_{x}V_{1-x}O_{3} thin films

The room temperature (300 K) electronic structure of pulsed laser deposited LaNi_{x}V_{1-x}O_{3} thin films have been demonstrated. The substitution of early-transition metal (TM) V in LaVO_{3} thin films with late-TM Ni leads to the decreasing in out-of-plane lattice parameter. Doping of Ni does not alter the formal valence state of Ni and V in LaNi_{x}V_{1-x}O_{3} thin films, divulging the absence of carrier doping into the system. The valence band spectrum is observed to comprise of incoherent structure owing to the localized V 3d band along with the coherent structure at Fermi level. With increase in Ni concentration, the weight of the coherent feature increases, which divulges its origin to the Ni 3d-O 2p hybridized band. The shift of Ni 3d-O 2p hybridized band towards higher energy in Ni doped LaVO_{3} films compared to the LaNiO_{3} film endorses the modification in ligand to metal charge transfer (CT) energy. The Ni doping in Mott-Hubbard insulator LaVO_{3} leads to the closure of Mott-Hubbard gap by building of spectral weight that provides the delocalized electrons for conduction. A transition from bandwidth control Mott-Hubbard insulator LaVO_{3} to negative CT metallicity character in LaNiO_{3} film is observed. The study reveals that unlike in Mott-Hubbard insulators where the strong Coulomb interaction between the 3d electrons decides the electronic structure of the system, CT energy can deliver an additional degree of freedom to optimize material properties in Ni doped LaVO_{3} films.Comment: 30 pages, 8 figure

Self-Powered Photodetector Fabricated from a Single-Charge-Transfer Complex Nanowire Grown In Situ between Prefabricated Electrodes on an Si3N4 Membrane

Power consumption in an electronic circuit is one ofthe seriouschallenges that need to be improved to achieve a durable future. Aphotodetector is one such electronic device that consumes a huge externalpower to operate. This motivated the researchers to concentrate onself-powered optical photodetectors that can operate without externalbias. On the contrary, building a device on a transparent film ornanomembrane has great importance in the field of electronic skins,and lightweight and intelligent wearables technology. Here, we havereported the fabrication and characterization of a self-power opticalphotodetector device based on a single Cu:7,7,8,8-tetracyanoquinodimethanenanowire (Cu:TCNQ NW) of length & SIM;500 nm and diameter & SIM;50nm. The NW photodetector device was fabricated on a silicon nitride(Si3N4) membrane window (size = 100 & mu;mx 100 & mu;m, membrane thickness & SIM;100 nm) to meet thedemands of a lightweight and transparent technology. The reportedself-powered single-NW photodetector exhibits excellent photoresponsivity(& SIM;5.5 A/W), high detectivity (& SIM; 7 x 10(7) Jones), outstanding external quantum efficiency (& SIM;1.6 x10(3)%), and a large on/off current ratio (& SIM;1.5 x10(2)) at 49 nW optical power. The analysis reveals thatthe contribution is due to photocarrier generation, radial built-in-fieldon the NW's surface, and barrier height reduction during illumination.Self-powered optical photodetectors, in particular, have enormouspotential as novel emerging self-driven optoelectronic devices

Effect of low temperature structural phase transitions in BaTiO3 on electrical transport through a metal-ferroelectric-metal multilayer of AuCr/BaTiO3/Nb:SrTiO3

In this paper we report an investigation of electronic transport through the metal-ferroelectric-metal (MFM) multilayer consisting of AuCr/BaTiO3/Nb:SrTiO3 over a temperature range of 100 K-300 K where BaTiO3 (BTO) shows a series of structural phase transitions leading to change of magnitude as well as the orientation of the polarization (P)over-right-arrow. We observed that the bias dependent barrier heights associated with the interfaces carry strong signature of the phase transitions in the BTO layer which lead to a strong temperature dependent asymmetric transport, when cooled down below room temperature. Specifically, it is observed that the temperature dependence is closely correlated to low temperature transitions in the BTO layer as revealed through the temperature dependent x-ray diffraction (XRD), capacitance as well as resistivity behavior of the BTO layer. There is substantial enhancement of the asymmetry in the device current that occurs at or close to temperatures T-2 similar to 190 K where BTO shows a crystallographic phase change to the low temperature rhombohedral phase. The temperature dependent changes occur due to barrier modulation at the interfaces of AuCr/BaTiO3 as well as BaTiO3/Nb:SrTiO3 that softens on cooling due to inhomogenities present there. The change in barrier on change of the bias direction has been observed below T-2 which arises from alignment of the polarization in-plane or out-of-plane as determined by tensile or compressive character of the in-plane strain in the BTO film. We also discuss the effect of space charge determined by the oxygen vacancies in the interface region, regulated by the applied bias

Floating Back-Gate Field Effect Transistor Fabricated Using a Single Nanowire of Charge Transfer Complex as a Channel

Metal–organic charge transfer complex (MOCT) material have good application potentials. We report a high carrier mobility in MOCT material Cu:tetracyanoquino­dimethane (Cu:TCNQ) single nanowire (NW). A novel floating back-gate field effect transistors is fabricated using Cu:TCNQ single NW of diameter ranging from ∼50 to 100 nm and length ∼1.0–2.0 μm as channel material. Floating gate is made of conducting Si (c-Si) electrically isolated from the environment by thermally grown SiO₂(100 nm thickness) all around it, which is an easy and inexpensive approach to reduce leakage current and improve the device performance. The devices can exhibit on/off current ratio of ∼10²–10⁴ at room temperature. Mobility of the NW channel as measured in different single NW devices is ∼4.3 × 10² to 1.2 × 10⁴ cm² V^–1 s^–1 which is the best reported mobility in such molecular materials. The observed high mobility has been proposed to arise from π–π stacking of the molecular orbitals of donor–acceptor type CT materials and good crystallinity and also small device size that reduces carrier scattering

Plasticity-mediated collapse and recrystallization in hollow copper nanowires: a molecular dynamics simulation

We study the thermal stability of hollow copper nanowires using molecular dynamics simulation. We find that the plasticity-mediated structural evolution leads to transformation of the initial hollow structure to a solid wire. The process involves three distinct stages, namely, collapse, recrystallization and slow recovery. We calculate the time scales associated with different stages of the evolution process. Our findings suggest a plasticity-mediated mechanism of collapse and recrystallization. This contradicts the prevailing notion of diffusion driven transport of vacancies from the interior to outer surface being responsible for collapse, which would involve much longer time scales as compared to the plasticity-based mechanism

Gated Photodetector with a Bipolar Response from Single-Crystal Halide Perovskite Using a Polymeric Electrolyte as the Gate Dielectric

In this report, we show that a gated optical detector working in the visible wavelength region can be made on single-crystal halide perovskite methylammonium lead bromide (CH3NH3PbBr3 or MAPB). A polymeric electrolyte (PEO/LiClO4) is used as the gate dielectric, which forms an electric double layer (EDL) at the electrolyte/MAPB interface, leading to high specific gate capacitance and enabling enhanced carrier induction at a low gate bias. The photoresponse of the detector can be enhanced significantly by a large factor (e.g., by a factor of 35) by a bias V-g of 10 V. The core gate operation is due to the field effect, and the detector shows the characteristics of a field effect transistor (FET) with bipolar nature, thereby operating with both polarities of the gate bias. This is enabled by the special feature of halide perovskites, that is, they have appreciable mobility for both types of carriers. It is established that the enhancement of the detector current response occurs due to the synergy of carriers created by illumination as well as the gate when they are present simultaneously, which modifies the near-band-edge trap states close to valence band maxima (VBM) and conduction band minima (CBM) and enhances the carrier mobility. The proposed synergy mechanism is validated by the gate-induced enhancement of the photoluminescence (PL) emission intensity and narrowing of the emission line

Surface/Interface Defect Engineering on Charge Carrier Transport toward Broadband (UV-NIR) Photoresponse in the Heterostructure Array of p-Si NWs/ZnO Photodetector

The surface/interface properties, especially interfacial states, have a key impact on overall carrier generation, recombination/transport, and/or collection proficiency for heterostructurebased photodetectors. This study demonstrates the significant enhancement of ultraviolet-near infrared (UV-NIR) (300-1100 nm) broadband photodetection in the heterostructure array of p-Si NWs/ZnO photodetectors with engineering of surface/interface charge carrier transportation under different processing conditions. In the case of a pulsed laser deposition (PLD)-grown photodetector, coupling of the subsidiary value of the defect state with the interfacial layer (Si-O-Zn) at the p-n junction reduces the charge carrier recombination, resulting in a large enhancement of transient photocurrent in the visible (Vis)-NIR region. However, in the case of a chemical solution deposition (CSD)-grown photodetector, plenty of oxygen vacancies (Vos) become the trap-assisted recombination centers by capturing of photoinduced carriers. The average value of responsivity (R) at 1 V bias for the PLD-grown detector is -5.5 A/W in the Vis-NIR (500-1100 nm) region, whereas in the UV region (<= 375 nm), the value of R reached -8 A/ W. The value of R in the PLD-grown detector is enhanced -102 folds in the UV region and -20 folds in the Vis-NIR region comparison with the CSD-grown detector. Further, carrier generation, trapping, and transport/recombination processes in the surface/interface are well illustrated to explain the dynamics of the charge carrier contributing to the photoresponse behavior in the UV-NIR broadband region