Low-Temperature Thermal Conductance in Superlattice Nanowire with Structural Defect

Using the scattering-matrix cascading method, we investigate the effect of structural defect on the acoustic phonon transmission and thermal conductance in the superlattice nanowire at low temperatures. In the present system, the phonon transmissions exhibit quite complex oscillatory behaviour. It is found that a lateral defect in an otherwise periodic structure significantly decrease the thermal conductance and completely washes away the transmission quantization. However, the appreciable transmission quantization survives in the presence of a longitudinal defect whereas a good quantization plateau of thermal conductance emerges below the universal level in a wide temperature range with the lateral defect.     

Nonequilibrium Green's function approach to phonon transport in defective carbon nanotubes

We have developed a new theoretical formalism for phonon transport in nanostructures using the nonequilibrium phonon Green's function technique and have applied it to thermal conduction in defective carbon nanotubes. The universal quantization of low-temperature thermal conductance in carbon nanotubes can be observed even in the presence of local structural defects such as vacancies and Stone-Wales defects, since the long wavelength acoustic phonons are not scattered by local defects. At room temperature, however, thermal conductance is critically affected by defect scattering since incident phonons are scattered by localized phonons around the defects. We find a remarkable change from quantum to classical features for the thermal transport through defective carbon nanotubes with increasing temperature.     

Infrared absorption by polar optical phonons in a CdS nanocrystal array consisting of quantum dots and quantum wires

Based on the results of experimental studies, it is shown that, when the effect of size quantization on the vibratory properties of CdS nanocrystals is insignificant, the absorption spectra of nanocomposites containing CdS quantum wires and quantum dots exhibit not only the peak associated with the transverse optical phonons but also two other pronounced peaks associated with electrostatic vibration modes. When the size of nanocrystals is so small that the effect of size quantization cannot be neglected, the structure of the reflection and transmission spectra becomes much more complicated because of the contribution of the mixed longitudinal-transverse phonon modes.     

Dissipative interaction and anomalous surface absorption of bulk phonons at a two-dimensional defect in a solid

We predict an extreme sensitivity to the dissipative losses of the resonant interaction of bulk phonons with a 2D defect in a solid. We show that the total resonant reflection of the transverse phonon at the 2D defect, described earlier without an account for dissipation, occurs only in the limit of extremely weak dissipation and is changed into almost total transmission by relatively weak bulk absorption. Anomalous surface absorption of the transverse phonon, when one half of the incident acoustic energy is absorbed at the 2D defect, is predicted for the case of “intermediate” bulk dissipation.     

边界条件对介电量子波导中声子输运性质的影响

运用散射矩阵方法，研究了Neumann边界条件和Dirichlet边界条件在低温下对结构不连续的纳米结构中的声学声子输运系数的影响.数值结果表明，当边界条件不同时，声学声子输运系数会有极大的不同；在一定的结构条件下，由于声子模与模的耦合作用，出现了共振透射和禁止频带.     

多通道石墨纳米带中弹性声学声子输运和热导特性

采用非平衡格林函数方法, 在保持总的能量输出通道中石墨链数不变的条件下, 研究并比较了并列的石墨纳米带通道中弹性声学声子输运和热导特性. 结果表明, 能量输出通道的增加能降低每个能量输出通道的热导; 与能量输入热库最近的能量输出通道热导最大, 最远的能量输出通道热导最小; 中间能量输出通道的热导性质与并列的各输出通道的结构参数密切相关, 最近和最远的能量输出通道的热导性质仅与各自能量输出通道的结构参数有关; 粗糙边缘结构能有效调节各通道的热导; 总的热导性质与能量输出通道石墨链数、能量输出通道数以及边缘结构粗糙程度密切相关.     

Titanium dioxide synthesized using titanium chloride: size effect study using Raman spectroscopy and photoluminescence

Titanium dioxide nanocrystals were prepared by the wet chemical method and characterized by X‐ray diffraction (XRD), transmission electron microscopy (TEM), Raman scattering (RS) and photoluminescence techniques. The XRD pattern shows the formation of single phase anatase structure of average sizes ∼7 nm (sample A) and ∼15 nm (sample B) for two samples. Additionally, TEM and RS were used to confirm the anatase crystal structure for both samples. The PL spectra show that the intensity of the sample A is more than that of sample B, which has been attributed to defect(s) and particle size variation. A modified phonon confinement model incorporating particle size distribution function and averaged dispersion curves for two most dispersive phonon branch (Γ‐X direction) have been used to interpret the size effect in Raman spectra. The obtained Raman peak shift and full width at half‐maximum agree well with the experimental data. Our observations suggest that the phonon confinement effects are responsible for a significant shift and broadening for the Raman peaks. Copyright © 2009 John Wiley & Sons, Ltd.     

Imaging of spatial structures in crystals with ballistic phonons

Ballistic phonon propagation in single crystals has been used for imaging defect structures in bulk material. We have realized this principle by means of low-temperature scanning electron microscopy. As model cases we have imaged laser drilled holes in single-crystalline sapphire and -quartz samples. Comparing the acoustic images with those obtained by optical transmission microscopy, we found that fine structure seen optically was well reproduced acoustically. In addition strain fields due to plastic deformations could clearly be imaged acoustically. The spatial resolution of our imaging technique is estimated to be a few micrometer. By using two or more phonon detectors, three-dimensional imaging of spatial inhomogeneities becomes possible.     

含双T形量子结构的量子波导中声学声子输运和热导

采用散射矩阵方法, 研究了含双T形量子结构的量子波导中声学声子输运和热导性质. 结果表明: 在极低温度, 双T形量子结构能增强低温热导; 相反地, 在相对较高的温度范围, 双T形量子结构能降低低温热导. 而在整个低温范围内, 增加散射区域最窄处的宽度能增强低温热导. 计算结果表明可以通过调节含双T形量子结构的量子波导结构来调控声子的输运概率和热导. 关键词： 声学声子输运 热导 量子结构     

Terahertz absorption by electrons and confined acoustic phonons in free-standing quantum wells

The acoustic phonon confinement in a free-standing quantum well (FSQW) results in an acoustic phonon energy quantization. Typical quantization energies are in the terahertz frequency range. Free electrons may absorb electromagnetic waves in this frequency range if they emit or absorb acoustic phonons. Therefore, the terahertz absorption reveals the characteristic features of the acoustic phonon spectrum in free-standing structures. We have calculated the absorption coefficient of an electromagnetic wave by free electrons in a FSQW in the terahertz frequency range. We took into account a time dependent electric field, an exact form of the acoustic phonon spectrum and eigenmodes, and electron interactions with confined acoustic phonons through the deformation potential. We demonstrate numerical results for GaAs FSQW of width 100 Å at low lattice temperatures in the frequency range 0.1-1 THz. The absorption coefficient exhibits several structures at frequencies corresponding to the lowest acoustic phonon modes. These features occur due to absorption of photons by electrons, which is accompanied by the emission of corresponding acoustic phonons.     

两耦合半无限超晶格中的局域界面声子-极化激元

采用转移矩阵方法，研究了含结构缺陷层的两耦合半无限超晶格(GaAs/AlAs)中的局域界面声子-极化激元模性质. 研究发现，含不同介电特性的缺陷超晶格结构中的局域界面声子-极化激元模在剩余射线区［ω_TO, ω_LO］的分布情况与数量存在不同，而且反对称模表现出不同的特征. 文中着重研究了缺陷层介电常数与角频率无关的缺陷超晶格，发现该结构中的局域界面声子-极化激元模对组分层的排列顺序与厚度、缺陷层的厚度以及横向波数有着不同程度的依赖.     

Ballistic phonon transport through a Fibonacci array of acoustic nanocavities in a narrow constriction

We investigate the ballistic phonon transport through a Fibonacci array of acoustic nanocavities in a narrow constriction of a semiconductor nanowire at low temperatures. It is found that the transmission spectrum of such a system consists of quasiband gaps and narrow resonances caused by the coupling of phonon waves. Both phonon transmission and thermal conductance exhibit the similarity due to the Fibonacci sequence structure. The similarity is sensitive to the number n and parameters of nanocavities. The results are compared with those in a periodic acoustic nanocavities.     

异侧非重叠三封闭端量子波导中的声子输运与热导

研究了异侧非重叠三封闭端量子波导中的声学声子传输和热导率性质。结果表明：由于激发模的产生,总传输系数在整数约化频率的时发生跳跃;各个激发模所产生的温度条件不一样,温度越高,被激发的模越多,并且高阶模对热导的影响较小;声子传输和热导性质与不连续结构的形状和位置有直接的关系,声子传输和热导性质对量子线的温度环境相当敏感。     

Tailoring electron–phonon interaction in nanostructures

Efficient design of optoelectronic devices based on electron intersubband transitions depends critically on the knowledge of the intersubband relaxation times which in turn, depends on electron scattering with LO and acoustic phonons. In this article the intersubband scattering time associated with electron–acoustic-phonon interaction has been discussed in terms of phonon mode quantization and phonon confinement with describing the acoustic phonon dispersion relation in detail by introducing the cut-off frequency for each mode. It has been shown that the quantization of acoustic phonon modes lead to an enhancement in electron–phonon scattering time in AlGaAs quantum well structures. Based on the presented model, a new tailoring method has presented to adjust the electron–phonon scattering time in intersubband-transition-based structures while keeping the electronic properties unaltered. Also, we illustrated that for a quantum well with subband energy separation of ∼30 meV, the intersubband scattering time with acoustic-phonon-assisted transitions could be tailored from ∼120 ps to increased value of ∼400 ps or reduced value of ∼45 ps by inserting a 1 nm-thickacoustically soft or hard layers, respectively, while keeping the same the initial energy separation.     

Cr doping induced structural, phonon and excitonic properties of ZnO nanoparticles

We report systematic study of structural, phonon and optical properties of Cr-doped ZnO nanoparticles. These particles are synthesized through simple sol–gel technique. Structural studies carried out by X-ray diffraction method, confirm that the prepared particles are in hexagonal wurtzite structure and lattice parameters change considerably while increasing the doping. Raman and Fourier transform infrared spectral studies show that the intensity of the phonon modes decreased and also blue shift due to ion doping, respectively. Apart from this, transmission electron microscopic studies show reduction in particle size where the particle diameters reduced from 36 to 11 nm. Optical absorption spectral measurements show a blue shift in the band-gap and increment in excitonic oscillator strength. Photoluminescence studies show doping altered the near-band edge emission but there is no change in the other emission bands which is due to oxygen vacancy, surface defects and surface dangling bonds.     

Double heterojunction lasers and quantum well lasers

The improved technology of compound semiconductor heterojunction preparation has resulted in very reliable CW, room temperature diode lasers for optical information read-out grown on p-type substrates on the one hand and very abrupt double heterojunction diode lasers based on quantum effects on the other hand. The influence of quantization effects on the emission wavelength, the threshold current and its temperature dependence are discussed. A distinction has been made between quantization due to strong magnetic fields giving rise to a one-dimensional electron gas (quantum wire) and quantization resulting from electrostatic and/or compositional changes (quantum well). The double heterojunction as a test structure to study carrier scattering into quantum wells, the phonon participation in the hot carrier relaxation process and optical flux guiding in graded heterojunctions have been emphasized.     

Optical phonons in Ge quantum dots obtained on Si(111)

The Raman light scattering from optical phonons of Ge quantum dots grown by molecular beam epitaxy on a Si(111) surface is studied. A series of Raman lines related to the quantization of phonon spectrum is observed. It is shown that phonon frequencies are adequately described in terms of the elastic properties and the dispersion of the optical phonons of bulk Ge. The strain experienced by the Ge quantum dots is estimated.     

Size-dependent phonon transmission across dissimilar material interfaces

In this paper, we study the size effects on the phonon transmission across material interfaces using the atomistic Green's function method. Layered Si and Ge or Ge-like structures are modeled with a variety of confined sizes in both transverse and longitudinal directions. The dynamical equation of the lattice vibration (phonon waves) is solved using the Green's function method and the phonon transmission is calculated through the obtained Green's function. Phonon transmission across a single interface of semi-infinite Si and Ge materials is studied first for the validation of the methodology. We show that phonon transmission across an interface can be tuned by changing the mass ratio of the two materials. Multi-layered superlattice-like structures with longitudinal size confinement are then studied. Frequency-dependent phonon transmission as a function of both the number of periods and the period thickness is reported. A converged phonon transmission after ten periods is observed due to the formation of phonon minibands. Frequency-dependent phonon transmission with transverse size confinement is also studied for the interface of Si and Ge nanowire-like structures. The phonon confinement induces new dips and peaks of phonon transmission when compared with the results for a bulk interface. With increasing size in the transverse direction, the phonon transmission approaches that of a bulk Si/Ge interface.     

结构相变的规范场理论

 本文首先从场论的观点出发，构造了完整晶体的拉氏函数——晶体声子场的拉氏函数。利用晶体声子场在局域群G=SO(3)下的对称性破缺，引入了缺陷规范场。借助于场论中的真空对称性自发破缺和缺陷规范场理论，很自然地把缺陷引到结构相变的研究中。我们给出了相变温度θ_c，入点的热容量的跃变值ΔC_v，及序参数Φ_i的计算公式。从公式中，明显可看出缺陷对相变温度、热容变化值及序参数的影响。另外，我们确定了缺陷规范场理论中的耦合常数g。最后，我们给出了由压力引起的结构相变（如冲击相变）的一种可能的机理。     

嵌入线型缺陷的石墨纳米带的热输运性质

采用非平衡格林函数方法研究了嵌入有限长、半无限长、 无限长线型缺陷的锯齿型石墨纳米带 (ZGNR)的热输运性质.结果表明, 缺陷类型和缺陷长度对ZGNR的热导有重要影响. 当嵌入的线型缺陷长度相同时, 包含t5t7线型缺陷的石墨纳米带比包含Stone-Wales线型缺陷的条带热导低. 对于嵌入有限长、同种缺陷的ZGNR, 其热导随线型缺陷的长度增加而降低, 但是当线型缺陷很长时, 其热导对缺陷长度的变化不再敏感.通过比较嵌入有限长、半无限长、无限长线型缺陷的ZGNR, 我们发现嵌入无限长缺陷的条带比嵌入半无限长缺陷的条带热导高, 而后者比嵌入有限长线型缺陷的条带热导高. 这主要是因为在这几种结构中声子传输方向的散射界面数不同所导致的. 散射界面越多, 对应的热导就越低. 通过分析透射曲线和声子局域态密度图, 解释了这些热输运现象. 这些研究结果表明线型缺陷能够有效地调控石墨纳米带的热输运性质. 关键词： 石墨烯 线型缺陷 热导