Thin-film Quantum Research Articles

Atomic-Resolution Analysis of 2-D and Thin Film Quantum Materials

Atomic-Resolution Analysis of 2-D and Thin Film Quantum Materials

Enhanced current generation in quantum-dot intermediate band solar cells through optimizing the position of quantum dot layers

We present a numerical study using a self-consistent drift-diffusion approach to characterize the effect of light trapping on the quantum efficiency (QE) of a thin-film quantum dot intermediate band solar cell (QD-IBSC). In the concept of the IBSCs, the photons with energies lower than the bandgap of the host material can be absorbed in QDs due to the presence of the IB, leading to high photocurrent and conversion efficiency. However, in general, the quality of the QD structure is degraded due to the accumulated strain when the number of QD layers stacked vertically is increased, which is required to increase light absorption. In the present work, we adopted a Fabry-Pérot (FP) resonance structure to achieve high absorption with fewer QD layers. We demonstrate numerically, in a InAs/GaAs-based QDSC structure, that an increase in the QE by 8 times at maximum is achievable by tuning the position of the QD layers inserted.

Highly Efficient, Surface Ligand Modified Quantum Dot Light‐Emitting Diodes Driven by Type‐Controllable MoTe2 Thin Film Transistors via Electron Charge Enhancer

AbstractThe development of transition metal dichalcogenides (TMDCs) and quantum dots (QDs) as the promising semiconductor and emitter is carried out in diverse applications. Despite superb research progress, study of emerging devices incorporated with TMDC thin film transistors (TFTs) and quantum dot light‐emitting diodes (QLEDs) is rarely reported. Herein, the QLEDs operation controlled by p (or n‐type) molybdenum ditelluride (MoTe2) TFTs with the realization of complementary type transistor is first demonstrated. In this study, molecular doping by poly‐L‐lysine (PLL) is adopted for a type conversion of MoTe2 TFTs and surface ligand modification is utilized for improvement of QLED performance. As a result, the PLL treatment achieves the outstanding type conversion of MoTe2 TFTs without any degradation of electrical properties, leading to securing reliable n or p‐type devices, thus, availability of complementary circuits. Furthermore, ligand modified QDs capped with octylamine result in balanced electron/hole injection in QLEDs, yielding improved current efficiency (ηA = 13.9 cd A−1) and longer lifetimes (T50 = 66 h at L0 = 3000 cd m−2). Lastly, MoTe2 TFTs demonstrate their capabilities to drive the QLEDs for the envisioned application including display backplane transistor with decent switching properties, immunity for generation of photocurrent, and operation stability.

Transport and magnetic properties of a superconducting closed loop containing a thin-film quantum interferometer

The current in a superconducting closed loop containing a thin-film interferometer with two point contacts (two Josephson-type weak links) has been measured as a function of the transport current and external magnetic field applied to interferometer. The differences between the transport and magnetic dependences of the system under study and similar characteristics for a superconducting closed loop containing an interferometer in the form of a single pressed point contact are discussed.

MoS2 hybrid heterostructure thin film decorated with CdTe quantum dots for room temperature NO2 gas sensor

Molybdenum disulfide (MoS2) is a very promising candidate for room temperature (RT) gas sensing applications. However, the limitation of synthesis techniques, incomplete recovery, and selectivity at RT are prime drawbacks. In this report, MoS2 nanoworms (NWs) thin film and CdTe quantum dots (QDs) decorated MoS2 NWs hybrid heterostructure thin film (CdTe QDs/MoS2 NWs) have been fabricated by scalable sputtering technique on the p-Si substrate and tested for RT NO2 gas sensing applications. The proposed CdTe QDs/MoS2 NWs hybrid heterostructure thin film sensor manifests an excellent sensor response (∼40 %), fast response time = 16 s, complete recovery (recovery time = 114 s) and highly selective towards 10 ppm NO2 at room temperature in comparison to pristine MoS2 NWs thin film sensor (response ∼26 %, response/recovery time ∼23 s/incomplete recovery). This superior gas sensing performance may be attributed to the combined effect of factors such as hybrid heteronanostructure with unique morphology, catalytic activity, synergistic effects, and p-n heterojunctions. The approach employed here may lead to the development of RT operable MoS2-based heterojunction gas sensors.

In situ measurement of temperature dependent picosecond resolved carrier dynamics in near infrared (NIR) sensitive device on action.

The carrier dynamics study of emerging near infrared (NIR) absorbing materials is an essential need to develop device technology toward enhanced NIR light harvesting. In this study, we have documented the design of an indigenously developed time correlated single photoncounting (TCSPC) system working in the NIR (900 nm-1700 nm) spectral region. The system is compatible to study transient photoluminescence of device samples under tunable bias voltages. The liquid nitrogen cooling and electrical heating of the sample chamber provides additional flexibility of temperature dependent study starting from -196 °C to 400 °C. As a model system to study, we have chosen a multilayer InAs/InGaAs/GaAs/AlGaAs dot in the dual well device sample as the thin film quantum dot heterostructures are of huge relevance in various NIR harvesting devices. We have investigated the detail carrier dynamics of the device sample using the transient photoluminescence upon varying temperature (80 K-300 K), varying emission energy and different bias voltages (0 V-15 V). The critical temperature (160 K) and critical bias (12 V) of achieving longest excited state lifetime has been mechanistically explained using various competing photophysical phenomena such as hole diffusion, energy relaxation, etc. The emission wavelength dependent study at below and above critical temperature further provides an insight into the dominance of carrier capture and thermal escape at the two different temperature zones. Along with the detail understanding of the carrier dynamics, the results can be helpful to get an idea of the electrical stability of the device and the operability temperature as well. The reasonable good resolution of the NIR TCSPC system and considerable good results ensure the future application of the same for other devices also.

Enhanced charge carrier generation by dielectric nanomaterials for quantum dots solar cells based on CdS-TiO2 photoanode

This paper reports an inclusive study of synthesis and characterization of quantum dots CdS embedded Pb0.85La0.15TiO3 (PLT15) dielectric nanocomposite materials for application in thin-film quantum dots solar cells (QDSSc). We have synthesized dielectric materials which act as a strong field-effect passivation, screened columbic attraction, back reflector and recombination inhibitor for solar cell fabrication. This material was deposited on FTO/TiO2 layer with a uniform mixed layer of nanorod (CdS)-nanoparticles (PLT15) which result in an increase in the SCs efficiency by times higher from the reference value. By adding dielectric materials in a prototype solar cell, an enhancement of 6.32mA/cm2 in short-circuit current and an absolute value of 4.5% enhancement in overall conversion efficiency are achieved. In particular, synthesis of materials, fabrication, structural, optical, electrical and stability of the prototype SCs are examined thoroughly. We also performed a detailed investigation of aging on the main characteristics of solar cells, such as efficiency, fill factor, short circuit current and open circuit voltage. It was observed that due to its cost effectiveness, stability, and enhance efficiency, this dielectric based hybrid thin film solar cells have high potential to be applied in industry in the near future.

Spin-polarized current in Zeeman-split d-wave superconductor/quantum wire junctions

Abstract We study a thin-film quantum wire/unconventional superconductor junction in the presence of an intrinsic exchange field for a d-wave symmetry of the superconducting order parameter. A strongly spin-polarized current is generated due to an interplay between Zeeman splitting of bands and the nodal structure of the superconducting order parameter. We show that strongly spin-polarized current is achievable for both metallic and tunnel junctions. This is because of the presence of a quantum wire (one-dimensional metal) in our junction. While in two-dimensional junctions with both conventional [F. Giazotto, F. Taddei, Phys. Rev. B 77 (2008) 132501] and unconventional [J. Linder, T. Yokoyama, Y. Tanaka, A. Sudbo, Phys. Rev. B 78 (2008) 014516] pairing states, highly spin polarized current takes place just for a tunnel junction. Also, the obtained spin-polarized current is tunable in sign and magnitude in terms of exchange field and applied bias voltage.

Highly Efficient Solid‐State Near‐Infrared Emitting Material Based on Triphenylamine and Diphenylfumaronitrile with an EQE of 2.58% in Nondoped Organic Light‐Emitting Diode

The development of efficient near‐infrared (NIR) emitting material is of current focus. Donor–acceptor (D–A) architecture has been proved to be an effective strategy to obtain narrow energy gap. Herein, a D–A‐type NIR fluorescent compound 2,3‐bis(4′‐(diphenylamino)‐[1,1′‐biphenyl]‐4‐yl)fumaronitrile (TPATCN) is synthesized and fully characterized. As revealed by theoretical calculations and photophysical experiments, TPATCN exerts the advantages of the relatively large dipole moment of the charge transfer state and a certain degree of orbital overlap of the local excited state. A highly mixed or hybrid local and charge transfer excited state might occur to simultaneously achieve both a large fraction of singlet formation and a high quantum efficiency in D–A system. TPATCN exhibits strong NIR fluorescence with the corresponding thin film quantum efficiency of 33% and the crystal efficiency of 72%. Remarkably, the external quantum efficiency of nondoped NIR organic light‐emitting diode (OLED) reaches 2.58% and remains fairly constant over a range of 100–300 mA cm−2, which is among the best results for NIR OLEDs reported so far.

The new ultra high-speed all-optical coherent streak-camera

In the present paper a new type of ultra high-speed all-optical coherent streak-camera was developed. It was shown that a thin resonant film (quantum dots or molecules) could radiate the angular sequence of delayed ultra-short pulses if a transverse spatial periodic distribution of the laser pump field amplitude has a triangle shape.

Enhance Efficiency of Solar Cell Using Luminescence PbS Quantum Dots Concentrators.

Thin film and sheet PbS quantum dots (QDs) concentrators were synthesized by sol-gel method using three different PbS concentrations (0.14, 0.2, and 0.4 mol%). The structure and morphology of the prepared PbS QDs were characterized by X-ray diffraction (XRD), Scan electron microscopy (SEM), and Transmission electron microscopy (TEM). The photostability of the PbS QDs concentrators under outdoor exposure to sunlight for 8 weeks was studied. The results showed that the PbS QDs sheet with PbS concentration (0.14 mol%) has the highest luminescence intensity. The sheet PbS QDs concentrator was used to couple with PV solar cell and the corresponding photoelectric conversion efficiency was measured under sun light illumination. I-V characteristics of the photovoltaic devices, both open circuit voltage and short circuit current were improved as compared to the device without collector. This indicates that the proposed technique is very useful for improving the efficiency of solar cell.

Probing buried magnetic interface structure with the quantum size effect in spin-dependent electron reflectivity

The quantum size effect (QSE) in electron reflectivity from Fe thin films grown on a W(110) surface precovered with a two monolayer Cu film has been investigated using spin polarized low energy electron microscopy. Spin-dependent QSE-induced oscillations in the reflected intensity occur with energy and film thickness. The series of intensity peaks that is observed identifies spin-dependent quantum well resonances in the Fe film that are sensitive to electronic band structure and details of the buried interface. Information about the spin-dependent unoccupied bands of the Fe film in the ΓΝ direction normal to the film plane is obtained by analyzing the observed quantum well resonance conditions. The spin-split bands that are determined are uniformly shifted downward by 1.7eV compared to bulk-like bands determined previously in Fe films on a bare W(110) substrate by the same method. Evidence is also obtained that the buried interface that defines the thin film quantum well boundary is located one layer above the W(110) surface. These results suggest that the Cu layer in direct contact with the substrate remains largely intact, but the weakly-bound second Cu layer mixes or segregates freely.

Gas phase chemical vapor deposition chemistry of triethylboron probed by boron–carbon thin film deposition and quantum chemical calculations

We present triethylboron (TEB) as a single-source precursor for chemical vapor deposition (CVD) of BxC thin films and study its gas phase chemistry under CVD conditions by quantum chemical calculations.

Thermoelectric properties of topological insulator Bi2Te3, Sb2Te3, and Bi2Se3 thin film quantum wells

The thermoelectric (TE) figure of merit ZT of topological insulator Bi2Te3, Sb2Te3, and Bi2Se3 thin film quantum wells is calculated for thicknesses below 10 nm, for which hybridization of the surface states as well as quantum confinement in the bulk are individually predicted to enhance ZT. Here, the question is addressed what ZT can be expected from coexisting surface and bulk states in such quantum wells. It is demonstrated that the parallel contributing bulk and surface channels tend to cancel each other out. This is because the surface-to-volume ratios of the thin films prevent the domination of transport through a single channel and because the individual bulk and surface ZTs are optimized at different Fermi levels.

Fabrication of Ni-doped TiO2 thin film photoelectrode for solar cells

TiO2 and Ni-doped TiO2 thin films have been prepared by sol–gel dip coating method. X-ray diffraction studies show that TiO2 and Ni-doped nanocrystalline TiO2 thin films are of anatase phase. The surface morphology of CdS quantum dot sensitized TiO2 thin film and CdS quantum dot sensitized Ni-doped TiO2 thin film were analysed by scanning electron microscopy. The absorption edge of TiO2 thin films shift towards longer wavelengths (i.e. red shifted) with Ni doping, which greatly enhances the light absorption of TiO2 in the visible region. The power conversion efficiency of CdS quantum dot sensitized Ni-doped TiO2 (CdS QDS-NT) based solar cell exhibited an efficiency of 1.33%, which is higher than that of CdS quantum dot sensitized TiO2 (CdS QDS-T) (0.98%) solar cells.

Change in properties of coated cadmium-tellurium quantum dots after heat treatment

The effects of heat treatment using an electronic furnace on the morphology of thin-film CdTe quantum dots (QDs) grown on glass substrates was investigated. The photoluminescence properties of CdTe QDs were found to depend on the size of QDs; photoemission was found to deteriorate, and sometimes, get completely lost with the change in QD size. Generally during the synthesis, QD size can be controlled by controlling the reflux time. However, since reflux temperature could not be increased beyond 100°C, we focused on heat treatment after synthesis. Therefore, the properties of fabricated QD solutions could be changed with additional heat treatment.

How important is nonlinear piezoelectricity in wurtzite GaN/InN/GaN quantum disk-in-nanowire LED structures?

Numerical investigation is carried out to compare the effects of linear and nonlinear piezoelectric fields on the electronic and optical properties of recently-proposed wurtzite GaN/InN/GaN quantum disk-in-wire LED structures. The computational framework employs a combination of fully atomistic valence force-field molecular mechanics and 10-band \(sp^{3}s\)*-SO tight-binding electronic band-structure models, and accurately captures the interplay between the long-range electro-mechanical fields and the quantum atomicity in the device. In particular, to model piezoelectricity in the wurtzite lattice, four different polarization models (based on the experimental and ab initio coefficients) have been considered in increased order of accuracy. In contrast to recent studies on thin-film quantum well structures, simulation results obtained in this work show that the nonlinear (second-order) piezoelectric contribution has insignificant effects on the overall electronic and optical properties in reduced-dimensionality (nanoscale) disk-in-wire LED structures.

Quantitative study of microtwins in GaP/Si thin film and GaAsPN quantum wells grown on silicon substrates

We report on the quantitative study of microtwins (MT) defects in the GaP/Si(001) thin film grown by Molecular Beam Epitaxy and the optical properties of GaAsP(N)/GaP(N) quantum wells grown on top of the GaP/Si pseudo-substrates. A 780nm photoluminescence at room temperature from the GaAsPN quantum wells is measured on silicon. Time-resolved photoluminescence has been performed and evidences the influence of non-radiative defects originated from the GaP/Si interface. The structural defects such as MT are quantitatively analyzed by synchrotron X-ray diffraction (XRD) combined with transmission electron microscopy (TEM) analyses. We show that the XRD measurements are in good agreement with TEM observation and reveal a strong contribution of MT in the [111] direction. The MT density appears to be directly correlated with the growth temperature.

Novel Synthesis of Chalcopyrite CuxInyS2 Quantum Dots with Tunable Localized Surface Plasmon Resonances

The burgeoning field of thin film quantum dot photovoltaics has made considerable strides toward efficient and inexpensive forms of third generation solar cells. However, these technologies have largely been based upon toxic metal-containing materials, limiting their foreseeable applications. Here we present a synthesis of nontoxic and stable CuxInyS2 quantum dots with tunable size and band gap. Interestingly, this synthesis leads to the presence of a broad-band and size-dependent absorption peak in the infrared (IR), attributed to localized surface plasmon resonances (LSPRs). Due to the sensitivity of their LSPR peak to quantum dot size and solvent refractive index, these quantum dots provide an attractive candidate for tunable plasmon resonance applications. And, if these LSPRs are found to be coupled with excitonic transitions, they may result in sizable increases in photovoltaic efficiency.

Abrupt Plasmonic Activation of Photoionization Rates in Quantum Dot Solids

We study emission of thin film quantum dot solids as a function of intensity of an illuminating laser field when they are deposited on substrates containing gold nanoparticles. Our results show that at a given intensity of this laser the plasmonically enhanced emission of such solids starts to decline abruptly, making them inefficient emitters. We show that such a process is caused by sudden activation of photoionization processes in such structures. For this, we demonstrate how the combined effects of the heat generated by the metallic nanoparticles and plasmons lead to the collapse of the electrostatic barrier (Coulomb blockage) responsible for suppression of such processes in quantum dot solids. The results highlight the strong dependency of plasmonic emission enhancement of such solids on the exciting light source intensity and demonstrate the limits where the impacts of plasmonic field enhancement can be harvested.