Size Uniformity Of Quantum Dots Research Articles

Highly luminescent and stable CH3NH3PbBr3 quantum dots with 91.7% photoluminescence quantum yield: Role of guanidinium bromide dopants

Abstract Although perovskite quantum dots (PQDs) have received considerable attention, defects in PQDs can significantly degrade the properties and device performance. In this study, we report on an effective strategy for synthesizing highly luminescent CH3NH3PbBr3 quantum dots (QDs) by a simple doping. To remove such defects, guanidinium bromide (GuBr) was doped into the CH3NH3PbBr3 QDs synthesized by the ligand-assisted reprecipitation technique. From XRD and TEM studies, the doping of GuBr into the QD lattices was verified. In addition, the surfaces of PQDs with and without GuBr dopants were analyzed by XPS to trace the metallic Pb acting as a major recombination center. The GuBr doping resulted in the size uniformity of QDs and effectively eliminated defects and metallic Pb, which enhanced the photoluminescence quantum yield (PLQY) through the inhibition of the non-radiative recombination pathway. Furthermore, the recombination dynamics in the QDs were examined by using time-resolved photoluminescence and fluorescence lifetime imaging to verify the role of GuBr dopants. By optimizing the amount of GuBr doping, the CH3NH3PbBr3 QDs with strong green emission achieved a maximum PLQY of 91.7%.

Boron doped Si rich oxide/SiO2 and silicon rich nitride/SiNx bilayers on molybdenum-fused silica substrates for vertically structured Si quantum dot solar cells

Vertically structured Si quantum dots (QDs) solar cells with molybdenum (Mo) interlayer on quartz substrates would overcome current crowding effects found in mesa-structured cells. This study investigates the compatibility between boron (B) doped Si QDs bilayers and Mo-fused silica substrate. Both Si/SiO2 and Si/SiNx based QDs bilayers were studied. The material compatibility under high temperature treatment was assessed by examining Si crystallinity, microstress, thin film adhesion, and Mo oxidation. It was observed that the presence of Mo interlayer enhanced the Si QDs size confinement, crystalline fraction, and QDs size uniformity. The use of B doping was preferred compared to phosphine (PH3) doping studied previously in terms of better surface and interface properties by reducing oxidized spots on the film. Though crack formation due to thermal mismatch after annealing remained, methods to overcome this problem were proposed in this paper. Schematic diagram to fabricate full vertical structured Si QDs solar cells was also suggested.

Controllable growth and optical properties of InP and InP/InAs nanostructures on the sidewalls of GaAs nanowires

The growth and optical properties of InP and InP/InAs nanostructures on GaAs nanowires are investigated. InP quantum well and quantum dots (QDs) are formed on the sidewalls of GaAs nanowires successively with increasing the deposition time of InP. The GaAs/InP nanowire heterostructure exhibits a type-II band alignment. The wavelength of the InP quantum well is in the range of 857–892 nm at 77 K, which means that the quantum well is nearly fully strained. The InP quantum dot, which has a bow-shaped cross section, exhibits dislocation-free pure zinc blende structure. Stranski-Krastanow InAs quantum dots are subsequently formed on the GaAs/InP nanowire core-shell structure. The InAs quantum dots are distributed over the middle part of the nanowire, indicating that the In atoms contributing to the quantum dots mainly come from the vapor rather than the substrate. The longest emission wavelength obtained from the InAs QDs is 1039 nm at 77 K. The linewidth is as narrow as 46.3 meV, which is much narrower than those on planar InP substrates and wurtzite InP nanowires, suggesting high-crystal-quality, phase-purity, and size-uniformity of quantum dots.

Critical size of substrate mesa for single quantum dot growth

Achieving periodicity and size uniformity of quantum dots is significant in improving the performance of quantum dot based devices. An efficient way to control the size and arrangement of quantum dots is to employ a patterned substrate in quantum dot growth. In this paper, the composition, position and shape transition of quantum dots are considered to obtain the minimum energy of the system. The least-energy principle is used here to estimate the critical size of the patterned substrate for single quantum dot growth. Numerical results are in good agreement with experiment, which can help one to control the arrangement of quantum dots by projection of the patterned substrate.

Influencing factors on the size uniformity of self-assembled SiGe quantum rings grown bymolecular beam epitaxy

The size uniformity of self-assembled SiGe quantum rings, which are formed by cappingSiGe quantum dots with a thin Si layer, is found to be greatly influenced by the growthtemperature and the areal density of SiGe quantum dots. Higher growth temperaturebenefits the size uniformity of quantum dots, but results in low Ge concentration as well asasymmetric Ge distribution in the dots, which induces the subsequently formedquantum rings to be asymmetric in shape or even broken somewhere in the ridge ofrings. Low growth temperature degrades the size uniformity of quantum dots, andthus that of quantum rings. A high areal density results in the expansion andcoalescence of neighboring quantum dots to form a chain, rather than quantum rings.Uniform quantum rings with a size dispersion of 4.6% and an areal density of7.8×108 cm − 2 are obtained at the optimized growth temperature of640°C.

Excitation-Induced Low Temperature Epitaxial Growth of Ge on Si(100)

ABSTRACTThe effect of laser-induced electronic excitations on the self-assembly of Ge quantum dots (QD) on Si(100)-(2x1) grown by pulsed laser deposition is studied. The experiment was conducted in ultrahigh vacuum. A Q-switched Nd:YAG laser (wavelength λ = 1064 nm, 10 Hz repetition rate) was split into two beams; one used to ablate a Ge target while the other to electronically excite the substrate. In situ reflection high-energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM) were used to study the morphology of the grown QDs. The dependence of the QD morphology on substrate temperature and ablation and excitation laser energy density was studied. Electronic excitation is shown to affect the surface morphology. For Ge coverage of 22 monolayer, it was observed that the excitation laser reduces the epitaxial growth temperature to 250 °C, a temperature at which no epitaxy is possible without excitation. Applying the excitation laser to the substrate during the growth changes the QD morphology and island density and improves the size uniformity of QDs at 390 °C. Surface diffusion measurement calculated from RHEED recovery curves show that the excitation-laser increases the surface diffusion of the Ge atoms. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms is involved.

자연 성장된 InAs/AlAs 양자점의 Photoreflectance 특성

MBE법으로 성장된 InAs/AlAs 양자점(quantum dots; QD) 구조의 광학적 특성을 photoreflectance(PR) 이용하여 조사하였다. Wetting layer(WL) 두께에 따른 전체 장벽의 폭이 달라짐에 따라 GaAs 완충층 및 WL 신호의 세기가 변화되었다. QD 층이 식각된 시료의 상온 PR측정 결과로부터 <TEX>$1.1{\sim}1.4\;eV$</TEX> 영역의 완만한 신호는 InAs QDs과 WL에 관련된 신호임을 알았다. 온도 <TEX>$450{\sim}750^{\circ}C$</TEX>범위에서 열처리 시켰을 때 WL층의 PR 신호가 red shift하였는데, 이는 열처리 후 InAs WL와 AlAs층 사이에 Al과 In의 내부 확산에 의해 양자점의 크기가 균일하게 재분포 되고, WL의 임계 두께가 증가하였음을 나타낸다. The optical characterization of self-assembled InAs/AlAs quantum dots(QD) grown by MBE were investigated using photoreflectance spectroscopy. The intensities of the signals of the GaAs buffer and wetting layer(WL) changed with the width of the WL layer. The PR spectrum for the sample, in which QDs layer were etched off at room temperature, indicated that the broadened signal ranging <TEX>$1.1{\sim}1.4\;eV$</TEX> was originated from InAs QDs and WL. The intensities of signals of GaAs buffer and the WL changed with the WL width. A red shift of the PR peak of WL are observed when the annealing temperatures range from <TEX>$450^{\circ}C$</TEX> to <TEX>$750^{\circ}C$</TEX>, which indicates that the interdiffusion between dots and capping layer is caused by improvement in size uniformity of QDs.