Journal of Materials Science: Materials in Electronics 2017 Volume 28( Issue 1) pp:839-844
Publication Date(Web):27 August 2016
DOI:10.1007/s10854-016-5598-7
Plasmons in metals have great impact on light emission, propagation, and detection in visible and infrared light wave frequencies. To explore plasmonic effect on the THz detection, both a backside-illuminated and a topside-illuminated blocking impurity band (BIB) THz detectors are developed and significant influence of plasmonic effect on the performance of BIB THz detectors is observed. The plasmonic effect in the heavily doped semiconductor layer of BIB THz detectors causes high reflectance of THz radiation which curtails the detection frequencies of the backside-illuminated BIB detectors. However, due to the advantages of flip-chip package and high quantum efficiency, the dark current, the responsivity, and the detectivity of the backside-illuminated detector shows superior characteristics to the topside-illuminated detector. The performance of the THz detector could be further improved with the suppression of the plasmonic effect.
Co-reporter:Weiguang Kong, Tao Ding, Gang Bi and Huizhen Wu
Physical Chemistry Chemical Physics 2016 vol. 18(Issue 18) pp:12626-12632
Publication Date(Web):06 Apr 2016
DOI:10.1039/C6CP00325G
Methylammonium lead-iodide (CH3NH3PbI3, hereafter referred to as MAPbI3) perovskite has emerged as a dazzling nova in the solar cell realm. To date, the surface physics of these materials is still puzzling, but in this work, we demonstrate that the optical dynamics in MAPbI3 is primarily determined by the surface states. Pb dangling bonds on the surface of MAPbI3 introduce shallow electronic states. The carrier localization effect for these electronic states is rather weak as the lifetimes of the carriers on the iodine-poor surface are comparative to those in the interior region of MAPbI3. In contrast, rich-iodine on the surface of MAPbI3 induces deep trap centers for the carriers, which are detrimental to long carrier diffusion lengths. It is further proved that the surface passivation, which surprisingly prolongs the carrier diffusion lengths, mainly works on the rich-iodine on the surface rather than the Pb dangling bonds. This better understanding of the surface physics could provide essential information for improving the performance of photoelectronic devices based on MAPbI3 perovskites.
The Journal of Physical Chemistry C 2016 Volume 120(Issue 14) pp:7606-7611
Publication Date(Web):March 21, 2016
DOI:10.1021/acs.jpcc.6b00496
Methylammonium lead iodide (MAPbI3) perovskite has emerged as a dazzling nova in the solar cell realm. However, the robustness or stability of the material exposed to different ambiences is a key issue. In this paper, resonance Raman spectroscopy is combined with surface and bulk crystal characterizations to interpret the oxygen intercalation phenomenon on the surface of MAPbI3. We observe that oxygen can intercalate into the frameworks of MAPbI3 with the assistance of laser excitation. By lowering down the pressure in the experimental chamber, the intercalated oxygen can be readily removed. X-ray photoelectron spectroscopy and X-ray diffraction characterizations suggest that Pb–O bonds are mainly formed on the surface of MAPbI3 but are constrained to avoid the formation of PbO compound. The quantum chemical calculation based on density functional theory supports the above conclusions. The understanding of oxygen intercalation in MAPbI3 shall benefit the improvement of stability of the important solar cell materials.
Co-reporter:Bingpo Zhang, Ping Lu, Henan Liu, Lin Jiao, Zhenyu Ye, M. Jaime, F.F. Balakirev, Huiqiu Yuan, Huizhen Wu, Wei Pan, and Yong Zhang
Nano Letters 2015 Volume 15(Issue 7) pp:4381-4386
Publication Date(Web):June 5, 2015
DOI:10.1021/acs.nanolett.5b01605
Quantum oscillations are observed in the 2DEG system at the interface of novel heterostructures, PbTe/CdTe (111), with nearly identical lattice parameters (aPbTe = 0.6462 nm, aCdTe = 0.648 nm) but very different lattice structures (PbTe: rock salt, CdTe: zinc blende). The 2DEG formation mechanism, a mismatch in the bonding configurations of the valence electrons at the interface, is uniquely different from the other known 2DEG systems. The aberration-corrected scanning transmission electron microscope (AC-STEM) characterization indicates an abrupt interface without cation interdiffusion due to a large miscibility gap between the two constituent materials. Electronic transport measurements under magnetic field up to 60 T, with the observation of Landau level filling factor ν = 1, unambiguously reveal a π Berry phase, suggesting the Dirac Fermion nature of the 2DEG at the heterostructure interface, and the PbTe/CdTe heterostructure being a new candidate for 2D topological crystalline insulators.
Co-reporter:Weiguang Kong, Zhenyu Ye, Zhen Qi, Bingpo Zhang, Miao Wang, Arash Rahimi-Iman and Huizhen Wu
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 25) pp:16405-16411
Publication Date(Web):27 May 2015
DOI:10.1039/C5CP02605A
Solution-processed hybrid perovskite of CH3NH3PbI3 (MAPbI3) exhibits an abnormal luminescence behavior at around the tetragonal–orthorhombic phase transition temperature. The combination of time resolved photoluminescence (PL), variable excitation power PL, and variable-temperature X-ray diffraction (XRD) allows us to clearly interpret the abnormal luminescence features in the phase transition region of MAPbI3. Both PL and XRD results unambiguously prove the coexistence of the tetragonal and orthorhombic phases of MAPbI3 in the temperature range of 150 to 130 K. The two luminescence features observed in the orthorhombic phase at T < 130 K originate from free excitons and donor–acceptor-pair (DAP) transitions, respectively. The comprehensive understanding of optical properties upon phase transition in MAPbI3 will benefit the development of new optoelectronic devices.
Co-reporter:Ruifeng Li, Zhenyu Ye, Weiguang Kong, Huizhen Wu, Xing Lin and Wei Fang
RSC Advances 2015 vol. 5(Issue 3) pp:1961-1967
Publication Date(Web):28 Nov 2014
DOI:10.1039/C4RA11012A
To understand the fundamental science of nanocrystal growth, the controllable synthesis and growth mechanism of dual size distributed PbSe quantum dots (QDs) are studied. The characterizations of high-resolution transmission electron microscopy (HR-TEM) and photoluminescence (PL) unambiguously demonstrate the dual size distribution of PbSe QDs. Thermodynamic stability of small QDs is confirmed by a controllable synthesis of temperature variation and kinetic perturbation with successive injection of precursors, suggesting a possible mechanism that a chemical-potential well may lead to the size separation. The control of growth temperature plays an important role in the realization of dual size distributed PbSe QDs. Further study of temporal evolution demonstrates the size refocusing of QDs at a higher temperature. Both kinetic perturbation and thermodynamic perturbation could facilitate QDs to overcome the potential barrier. Understanding this mechanism is of significance for the controllable synthesis and applications of PbSe QDs.
Journal of Luminescence 2014 Volume 148() pp:238-242
Publication Date(Web):April 2014
DOI:10.1016/j.jlumin.2013.12.007
•The dispersibility in water of as-prepared carbon dots is effectively improved by the addition of acid, alkali or metal ions.•The effect of hydrolysis on the optical properties of the carbon dots is studied.•The luminescent carbon dots show a pH-sensitive fluorescence and absorption property.•The addition of bivalent copper ions in the post-treated carbon dots offers a high color contrast for visual colorimetric assays for pH measurement.•The effect of surface defects and ligands on the performances of the carbon dots is also explored.Carbon dots with unique characters of chemical inertness, low cytotoxicity and good biocompatibility, demonstrate important applications in biology and optoelectronics. In this paper we report the optical properties of pH-sensitive carbon dots with different surface modifications. The as-prepared carbon dots can be well dispersed in water by modifying with acid, alkali or metal ions though they tend to form a suspension when being directly dispersed in water. We find that the carbon dots dispersed in water show a new emission and absorption character which is tunable due to the pH-sensitive nature. It is firstly proved that the addition of bivalent copper ions offers a high color contrast for visual colorimetric assays for pH measurement. The effect of surface defects with different modification on the performances of the carbon dots is also explored with a core–shell model. The hydro-dispersed carbon dots can be potentially utilized for cellular imaging or metal ion probes in biochemistry.
Co-reporter:Yong-Yue Chen;Xi-Kun Cai;Zhen-Yu Ye;Xiong Wang
Journal of Electronic Materials 2013 Volume 42( Issue 8) pp:2459-2463
Publication Date(Web):2013 August
DOI:10.1007/s11664-013-2589-9
The effects of growth temperature and annealing on the physical properties of Zn3Sn2O7 thin films were investigated in this work. The Zn3Sn2O7 thin films were deposited on glass substrates by radio frequency (rf) magnetron sputtering. It is found that the films are amorphous regardless of the growth temperature. The film grown at room temperature shows the highest mobility of 8.1 cm2 V−1 s−1 and the lowest carrier concentration of 2.0 × 1015 cm−3. The highest carrier concentration of 1.6 × 1019 cm−3 is obtained at the growth temperature of 250°C. Annealing treatment of the Zn3Sn2O7 thin films resulted in increases of carrier concentration and mobility. The average transmittance of the as-deposited and annealed films reaches 80%. By using a Zn3Sn2O7 thin film as the channel and a Ta2O5 thin film as the insulating layer, we fabricated transparent Zn3Sn2O7 thin-film transistors with field-effect mobility of 21.2 cm2 V−1 s−1, on/off current ratio of 105, threshold voltage of 0.8 V, and subthreshold swing of 0.8 V/decade.
Localized surface plasmons mediated photon energy conversion in ZnO/Ag/ZnO nanostructures has been investigated. Ag nano islands immersed in ZnO cause a great enhancement of visible light emission where silicon solar cells have maximum response, compared to that of ZnO single layer or Ag/ZnO bilayer structures. It is found that the visible light emission enhancement ratio increases with the thickness of the ZnO capping layer and an enhancement ratio of tenfold for 100 nm ZnO is realized. Furthermore, the intensity and peak wavelength of visible light emission are also influenced by the sizes of Ag islands. The observation of photoluminescence from ZnO/Ag/ZnO nanostructures is interpreted with theoretical calculation of localized surface plasmon resonance. The observed large enhancement of visible light emission may have potential applications in high efficiency solar cells by conversion of ultraviolet to visible light.
The Journal of Physical Chemistry C 2012 Volume 116(Issue 20) pp:11283-11291
Publication Date(Web):May 3, 2012
DOI:10.1021/jp210269m
In this paper, we report distinct enhancement of surface-state emissions (SSEs) of colloidal CdSe quantum dots (QDs) via coupling to localized plasmons (LPs) in Ag nanostructures. The roles of oleic acid (OA) ligand on QDs in the formation of Ag nanostructures and in the intense enhancement of SSEs of CdSe QDs are explored. We find that OA ligand on CdSe QDs plays a critical role in modifying the morphology of the contacted Ag, which consequently impacts the coupling of QD emitters with LPs in Ag. A systematic study of size effect of QDs on coupling of SSEs with LPs shows that as-deposited small Ag particles favorably enhance the SSEs of small-size CdSe QDs. The OA ligand on QDs yields better Ag crystallinity and clear corners during the annealing process; therefore, it promotes reshaping of small Ag particles into larger ones, favorable to enhance the SSEs of large-size CdSe QDs. The annealed QDs/Ag hybrid structures are more stable than the unannealed ones due to the loss of the OA ligand in the heating process. The selective coupling of QD emitters with LPs in Ag nanostructures allows feasible realization of microscale fluorescent color patterns. The approach of OA-assisted modification of plasmonic properties of Ag nanostructures provides a new route to synthesizing bright luminescence materials and devices that use colloidal QDs.
Regular shape defects on the surface of PbTe thin films grown by molecular beam epitaxy (MBE) were studied by scanning electron microscope (SEM). Two types of regular shape defects were observed on Te-rich PbTe films grown at substrate temperature T ≥ 235 °C with a beam flux ratio of Te to PbTe (Rf) to be 0.5 and at 280 °C with a Rf ≥ 0.4, which include cuboids and triangular pyramids. The formation mechanism of the observed regular shape defects is interpreted as following: They are the outcome of fast growth rate along {1 0 0} crystal planes that have the lowest surface energy and the enclosure of the {1 0 0} crystal planes. The formation of the regular shape defects in the growth of PbTe needs appropriate substrate temperature and Te-rich ambience. However, when Rf is decreased low enough to make the films slightly Pb-rich, triangular pits that originate from the insufficient glide of the threading dislocations along the main 〈1 1 0〉 {1 0 0} glide system of PbTe in Cottrell atmosphere, will be the main feature on the film surface.
Thin Solid Films 2011 Volume 519(Issue 10) pp:3254-3258
Publication Date(Web):1 March 2011
DOI:10.1016/j.tsf.2010.12.022
Considering practical applications in electronic devices, we studied the growth of In2O3 thin films on amorphous glasses by magnetron sputtering at room temperature and annealing effect on the structural and electrical properties. The vacuum annealed In2O3 thin films display a grain size enlargement and preferential orientation. Electrical characterization shows that the vacuum annealed In2O3 thin films exhibit a significant enhancement of both electron density and mobility, while air ambient annealing leads to a remarkable drop. The mechanism of the electrical characteristic changes in In2O3 thin films by annealing is explored by using different scattering mechanisms. Finally, a thin film transistor device using vacuum annealed In2O3 nano-meter thin films as active channel material is demonstrated.
The band offset at the interface of PbTe/Ge (1 0 0) heterojunction was studied by the synchrotron radiation photoelectron spectroscopy. A valence band offset of ΔEV = 0.07 ± 0.05 eV, and a conduction band offset of ΔEC = 0.27 ± 0.05 eV are concluded. The experimental determination of the band offset for the PbTe/Ge interface should be beneficial for the heterojunction to be applied in new optoelectronic and electronic devices.
Journal of Zhejiang University-SCIENCE A 2008 Volume 9( Issue 1) pp:137-142
Publication Date(Web):2008 January
DOI:10.1631/jzus.A071350
The combined characterizations of mobility and phonon scattering spectra allow us to probe hole transport process in epitaxial PbSe crystalline films grown by molecular beam epitaxy (MBE). The measurements of Hall effect show p-type conductivity of PbSe epitaxial films. At 295 K, the PbSe samples display hole concentrations of (5∼8)×1017 cm−3 with mobilities of about 300 cm2/(V·s), and at 77 K the hole mobility is as high as 3×103 cm2/(V·s). Five scattering mechanisms limiting hole mobilities are theoretically analyzed. The calculations and Raman scattering measurements show that, in the temperatures between 200 and 295 K, the scattering of polar optical phonon modes dominates the impact on the observed hole mobility in the epitaxial PbSe films. Raman spectra characterization observed strong optical phonon scatterings at high temperature in the PbSe epitaxial films, which is consistent with the result of the measured hole mobility.
PbSe thin films on BaF2 (1 1 1) were grown by molecular beam epitaxy with different selenium beam flux. Evolution of PbSe surface morphologies with Se/PbSe beam flux ratio (Rf) has been studied by atomic force microscopy and high-resolution X-ray diffraction. Growth spirals with monolayer steps on PbSe surface are obtained using high beam flux ratio, Rf ≥ 0.6. As Rf decreases to 0.3, nano-scale triangle pits are formed on the surface and the surface of PbSe film changes to 3D islands when Rf = 0. Glide of threading dislocations in 〈1 1 0〉{1 0 0}-glide system and Pb-rich atom agglomerations are the formation mechanism of spiral steps and triangle pits. The nano-scale triangle pits formed on PbSe surface may render potential applications in nano technology.
Cubic MgxZn1−xO thin films were deposited on amorphous silicon dioxide substrate by reactive electron beam evaporation (REBE) at low temperature (250 °C). The characterizations of crystalline structure and morphology of the ternary films demonstrated that the cubic MgxZn1−xO films are of highly (0 0 1) orientation and have uniform surface. The cubic Mg0.83Zn0.17O film deposited on quartz demonstrates wide band gap (6.45 eV) and has very high transparency (>95%) in broad wavelength range from ultraviolet (0.3 μm) to mid-infrared light (5.5 μm). The refractive indices for the cubic MgxZn1−xO decrease as Mg fraction increases. The characters of low optical absorption in broad wavelength range and feasibility of changing refractive index by Mg fraction variation in the ternary MgxZn1−xO films could render potential applications in integrated optical devices.
Journal of Crystal Growth (15 June 2015) Volume 420() pp:17-21
Publication Date(Web):15 June 2015
DOI:10.1016/j.jcrysgro.2015.03.031
•Molecular beam epitaxy of PbTe films on GaAs using CdTe/ZnTe buffers.•Orientation change of PbTe films from (211) of GaAs substrates.•Observation of optical phonons and mid-infrared luminescence from PbTe films.•n-type PbTe with electron density and mobility data.Narrow-gap semiconductor PbTe has exhibited versatility in both mid-infrared optoelelctronics and thermoelectrics. However, the absence of commercially obtainable PbTe crystal substrates limits its wide applications. In this paper, heteroepitaxy of high-quality PbTe crystal on GaAs(211) using CdTe/ZnTe buffers by molecular beam epitaxy is presented for the first time. Optimal growth parameters have been obtained by both in-situ and ex-situ characterizations. In-situ reflection high-energy electron diffraction observed a transition of growth mode from 2D to 3D, which is in agreement with the results of atomic force microscope and scanning electron microscope characterizations. High resolution X-ray diffraction revealed that the growth of PbTe crystal is along [531] direction which is different from the [211] substrate orientation. Multiple phonon modes related to PbTe were observed by Raman scattering while mid-infrared light emission from epitaxial PbTe is observed at a peak of 3.5 μm by photoluminescence. Different from PbTe grown on BaF2(111), n-type conductivity with electron densities of ~5×1017 cm−3 and mobilities of 675 cm2/V s at room temperature and 4300 cm2/V s at 2 K is observed. The high quality PbTe grown on GaAs(211) substrates using CdTe/ZnTe buffers renders promising applications in both optoelectronics and thermoelectrics.
Co-reporter:Weiguang Kong, Zhenyu Ye, Zhen Qi, Bingpo Zhang, Miao Wang, Arash Rahimi-Iman and Huizhen Wu
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 25) pp:NaN16411-16411
Publication Date(Web):2015/05/27
DOI:10.1039/C5CP02605A
Solution-processed hybrid perovskite of CH3NH3PbI3 (MAPbI3) exhibits an abnormal luminescence behavior at around the tetragonal–orthorhombic phase transition temperature. The combination of time resolved photoluminescence (PL), variable excitation power PL, and variable-temperature X-ray diffraction (XRD) allows us to clearly interpret the abnormal luminescence features in the phase transition region of MAPbI3. Both PL and XRD results unambiguously prove the coexistence of the tetragonal and orthorhombic phases of MAPbI3 in the temperature range of 150 to 130 K. The two luminescence features observed in the orthorhombic phase at T < 130 K originate from free excitons and donor–acceptor-pair (DAP) transitions, respectively. The comprehensive understanding of optical properties upon phase transition in MAPbI3 will benefit the development of new optoelectronic devices.
Co-reporter:Weiguang Kong, Tao Ding, Gang Bi and Huizhen Wu
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 18) pp:NaN12632-12632
Publication Date(Web):2016/04/06
DOI:10.1039/C6CP00325G
Methylammonium lead-iodide (CH3NH3PbI3, hereafter referred to as MAPbI3) perovskite has emerged as a dazzling nova in the solar cell realm. To date, the surface physics of these materials is still puzzling, but in this work, we demonstrate that the optical dynamics in MAPbI3 is primarily determined by the surface states. Pb dangling bonds on the surface of MAPbI3 introduce shallow electronic states. The carrier localization effect for these electronic states is rather weak as the lifetimes of the carriers on the iodine-poor surface are comparative to those in the interior region of MAPbI3. In contrast, rich-iodine on the surface of MAPbI3 induces deep trap centers for the carriers, which are detrimental to long carrier diffusion lengths. It is further proved that the surface passivation, which surprisingly prolongs the carrier diffusion lengths, mainly works on the rich-iodine on the surface rather than the Pb dangling bonds. This better understanding of the surface physics could provide essential information for improving the performance of photoelectronic devices based on MAPbI3 perovskites.
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