Liangliang Chen

An investigation of the optical properties of disordered silicon nanowire mats

Optical reflectance spectra of three disordered silicon nanowire mats with average diameters of 40, 60, and 80 nm are investigated both experimentally and theoretically. The total hemispherical reflectance spectra from 200 to 1600 nm wavelength are first measured. All three samples exhibit reflectance about 15% to 20% within the ultraviolet band. As the wavelength becomes longer, the reflectance will first increase to around 50% and then decrease to below 20%. Such reflectance spectra are attributed to the combined effect of silicon dielectric function, the nanowire geometry, and the volume fraction of the mats. An analytical method based on Mie scattering theory and two-flux model is proposed to predict the reflectance spectra of the NW mats using only the physical quantities including dielectric function and structural parameters of the nanowire mats. The experimental reflectance spectra can be well reproduced by this method.

Effects of rapid thermal processing and pulse-laser sintering on CdTe nanofilms for photovoltaic applications

Effects of rapid thermal annealing (RTA) and dual compression-pulse-laser sintering (compression-PLS) on photovoltaic, CdTe nanowire (NW) and quantum dot (QD) films are investigated. Unlike regular furnace annealing, RTA involves raising the temperature of a substrate’s atmosphere by several hundred degrees in a matter of seconds, letting it sit for 30 to 120 seconds, then cooling it back to T0. To the best of our knowledge, such treatments of CdTe nanocrystal (NC) films have not been documented. In compression-PLS, a large pressure (MPa) is applied to a film through a laser-pulsing mechanism. Next, a high-energy, high frequency laser beam is pulsed onto it for sintering. During the compression, we used a single pulse of 5 nanoseconds. For the sintering, we used a 7.05 mJ beam for two pulses, at 25 ns per pulse. Such parameters were determined from SEM and other preliminary film characterization results. Morphology, material content, and conductivity of the films are analyzed before and after treatment using tunneling and scanning electron microscopy, EDS, and two-probe measurements, respectively. This study provides new knowledge regarding the morphological and structural outcomes of RTA and compression-PLS on CdTe nanoparticle films. Furthermore, RTA and compression-PLS can increase the film electrical conductivity by improving their contact with each other. We found that RTA partially sinters the film and enhances inplane current density by a factor of ~1.7, for a values on the order of ~10-7A/cm2. Compression-PLS successfully sinters the NW film and improves current density up to a factor of ~167, for values on the order of ~10-5 A/cm2. On the other hand, QD films do not exhibit current density improvement with treatments. These values remain on the order of ~10-7 A/cm2. The resistivities of the sintered NW films reach as low as 6.7*106 Ω*cm, while the RTA’d NW film has a resistivity on the order of 108 Ω*cm. These values are comparable to values of bulk and thin-film CdTe: single crystalline, undoped CdTe resistivity values range from 105 to 108 Ω*cm,8,9 while polycrystalline thin-film values range from 104 to 106 Ω*cm.11,12 The QD films also have comparable resistivities to these results, albeit on the higher side.

Welding of Semiconductor Nanowires by Coupling Laser-Induced Peening and Localized Heating

We demonstrate that laser peening coupled with sintering of CdTe nanowire films substantially enhances film quality and charge transfer while largely maintaining basic particle morphology. During the laser peening phase, a shockwave is used to compress the film. Laser sintering comprises the second step, where a nanosecond pulse laser beam welds the nanowires. Microstructure, morphology, material content, and electrical conductivities of the films are characterized before and after treatment. The morphology results show that laser peening can decrease porosity and bring nanowires into contact, and pulsed laser heating fuses those contacts. Multiphysics simulations coupling electromagnetic and heat transfer modules demonstrate that during pulsed laser heating, local EM field enhancement is generated specifically around the contact areas between two semiconductor nanowires, indicating localized heating. The characterization results indicate that solely laser peening or sintering can only moderately improve the thin film quality; however, when coupled together as laser peen sintering (LPS), the electrical conductivity enhancement is dramatic. LPS can decrease resistivity up to a factor of ~10,000, resulting in values on the order of ~105 Ω-cm in some cases, which is comparable to CdTe thin films. Our work demonstrates that LPS is an effective processing method to obtain high-quality semiconductor nanocrystal films.

Effects of nanocrystal shape and size on the temperature sensitivity in Raman thermometry

The effects of CdSe nanocrystal (NC) shape and size on the temperature sensitivity of the Raman shift have been investigated, for the interest of Raman thermometry using NCs. For spherical CdSe NCs of diameters 2.8 nm, 3.6 nm, and 4.4 nm, the temperature sensitivities are −0.0131 cm−1/K, −0.0171 cm−1/K, and −0.0242 cm−1/K, respectively. This trend indicates that as the diameter increases, the effect of increasing phonon anharmonicity dominates over the effect of the decreasing thermal expansion coefficient. On the other hand, triangular NCs with a size of 4.2 nm and elongated NCs of a dimension of 4.6 nm by 14 nm show temperature sensitivities of −0.0182 cm−1/K and −0.0176 cm−1/K, respectively. This trend indicates that in non-spherical shape NCs, the effect of decreasing thermal expansion coefficient dominates over the effect of slightly increasing phonon anharmonicity. The selection of NCs for Raman thermometry should depend on the specific requirements of temperature sensitivity, spatial resolution, an...

Study of Interesting Length and Temperature Effect on Ultrafast Electron Relaxation in CdSe Nanorods

Time-domain non-adiabatic ab initio simulations are performed to study the phonon-assisted hot electron relaxation dynamics in CdSe QD, EQD and LQD, which are of the same diameter but an increasing length along c axis. Our work shows that both the length and system temperature have a strong impact on the electronic properties and electron relaxation dynamics of the CdSe QRs. Higher frequency phonons are excited and scattered with electrons at higher temperatures. The band gap shows a negative dependence on the temperature. The band gap decreases and the electron and hole density of states increase with increasing the length. However, not all the properties studied here vary with the length in a straight way. The band gap shows a stronger negative temperature dependence for the EQD than the QD and LQD. The electron-phonon couples stronger in the EQD than the QD and LQD. The hot electron relaxation proceeds faster in the EQD than the QD and LQD. Furthermore, the hot electron decay rate varies linearly with the average electron density of states and this linear relationship can be well described by the Fermi’s golden rule and of practical use in predicting the hot electron decay rate with the knowledge of the average NA coupling and electron density of states.Copyright

Size Dependent Raman Spectra of Bismuth Telluride Nanocrystals Synthesized via a One-Step Wet Chemistry Method

High-crystallinity 0D, 1D and 2D Bi2Te3 nanocrystals have been synthesized using the pyrolysis of organometallic compound method. The growth process of Bi2Te3 nanocrystals was revealed by transmission electron microscopy (TEM) images. Samples synthesized at the temperature of 35°C show a dominant morphology of 0D nanoparticle or 1D nanorod, while samples synthesized at the temperature above 75°C show a dominant morphology of 2D nanoplate. Phonon vibrational behavior was investigated by Raman spectroscopy. 2D nanoplates show similar Raman features to few-quintuple thick Bi2Te3 layers, while 0D and 1D nanostructures show a blueshifted A1g2 mode and a much stronger A1u mode. This is the report about Raman spectra obtained on small Bi2Te3 nanoparticle and nanorod whose size is within the strong quantum confinement region.© 2012 ASME

Shape and Temperature Dependence of Hot Carrier Relaxation Dynamics in Spherical and Elongated CdSe Quantum Dots

Time-domain non-adiabatic ab initio simulations are performed to study the phonon-assisted hot electron relaxation dynamics in CdSe spherical quantum dots (QDs) and elongated quantum dots (EQDs). EQDs have a narrower band gap and denser electron and hole energy states than QDs. As temperature increases, band gap values will become smaller due to thermal expansion effect. Also more phonons are excited to scatter with electrons and thus result in a higher relaxation rate for hot electrons. Besides, it is also found in our simulation that hot electron relaxation rate in EQDs has a weaker temperature dependence than in QDs, which could be attributed to the larger thermal expansion in EQDs.