Raman spectra of structures with CdTe-, ZnTe-, and CdSe-based quantum dots and their relation to the fabrication technology

Quantum-dot structures based on the CdTe, ZnTe, and CdSe semiconductors are prepared by molecular-beam epitaxy, colloid chemistry methods, and ball milling, and their Raman spectra are studied. Localized longitudinal phonons are observed in all spectra. The dependence of the localized phonon frequency on the thickness of the ZnTe barrier in CdTe/ZnTe quantum-dot superlattices is used to derive the dispersion relation for longitudinal phonons in ZnTe. The Raman spectra of ensembles of colloidal quantum dots differ from the spectra of the other objects by the absence of tellurium bands and a strong intensity of the longitudinal phonon band of CdTe. It is revealed that the spectra depend on the technology employed to prepare quantum-dot structures.     

Manifestation of interaction effects in Raman spectra of CdTe quantum-dot layers with ZnTe narrow barriers

The Raman spectra of superlattices consisting of layers of CdTe self-assembled quantum dots separated by ZnTe narrow barriers with thicknesses of 10 and 5 monolayers are investigated. It is found that, apart from the bands previously observed at frequencies of ～120 and ～140 cm^?1 for samples with thicker barriers (25 and 12 monolayers), the Raman spectra exhibit a band at ～147 cm^?1 in the frequency range of CdTe vibrational modes. This band is attributed to a symmetric vibrational mode of a pair of quantum dots with oppositely directed oscillations of the dipole moments. It is this type of vibrational mode in the material surrounding the ZnTe quantum dot that accounts for the shift of the band at ～200 cm^?1 near the LO mode of ZnTe vibrations toward lower frequencies.     

Raman scattering in multilayer structures with CdTe quantum dots in ZnTe

Raman spectra in superlattices composed of layers of self-assembled CdTe quantum dots separated by ZnTe barriers are investigated. As the barrier thickness increases, a high-frequency shift of all peaks is observed, which is explained by a decrease in the lattice constant averaged over the volume of the entire structure. Peaks are found at a CdTe TO mode frequency of 140 cm^?1 and also at 120 cm^?1. The first peak is assigned to the symmetric Coulomb (interface) mode of the quantum dot material, and the low-frequency peak is assigned to the symmetric mode of the phonons captured in the quantum dot. This combination of modes in structures with quantum dots has not been observed previously.     

Manifestation of CdTe quantum dots and interdiffusion in IR reflection spectra of CdTe/ZnTe structures with quantum dots

The IR reflection spectra of structures that consist of alternating layers of CdTe quantum dots and ZnTe barriers and are grown on ZnTe and CdTe/ZnTe buffer layers on GaAs(001) substrates are studied. The spectra are processed using dispersion analysis, and the parameters of the oscillators are determined. In the spectra, quantum dots manifest themselves in the form of a broad band at a frequency close to the frequency of the Fröhlich mode. It is revealed that the spectra contain features associated with the interdiffusion of the CdTe and ZnTe compounds, i.e., bands attributed to the local mode of Zn atoms in the CdTe compound and two gap modes of Cd atoms in the ZnTe compound.     

Long-wavelength IR spectroscopy of ZnTe/CdTe superlattices with quantum dots

This paper reports on the results of investigations of the lattice IR reflection spectra of ZnTe/CdTe multilayer superlattices with CdTe quantum dots grown by molecular-beam epitaxy on a GaAs substrate with a CdTe buffer layer. It is found that the lattice IR reflection spectra of the studied structures exhibit three intense bands associated with vibrational excitations in the GaAs substrate, ZnTe barriers separating the layers with CdTe quantum dots, and the CdTe buffer layer. An analysis of the reflection bands and shifts in the phonon frequencies has revealed internal elastic stresses both in the surface layer of the GaAs substrate and in the ZnTe barriers. It is established that elastic stresses undergo relaxation in the separating ZnTe layers with an increase in their thickness. An additional mode observed in the reflection spectra is explained by manifestations of ZnTe-like vibrations in the ZnCdTe alloy due to interdiffusion of Cd and Zn at the interfaces.     

CdTe量子点的光谱特性及其应用

研究了水相CdTe量子点的共振散射光谱、荧光光谱和吸收光谱特性。结果表明,随着量子点粒径(d)的增大,CdTe量子点的荧光峰(λ_F)发生红移,吸收峰也发生红移,且吸收峰(λ_A)的峰形变宽、吸光度(A)降低,λ与ln(d)均存在较好的线性关系。其函数关系为λ_A =126.74 ln(d)+395.92和λ_F=155.01 ln(d) +415.5。共振散射光谱研究表明, 共振散射波长λ_R与CdTe量子点粒径（3.8～8.6 nm）的对数存在较好的线性关系,线性回归方程为λ_R=148.37 ln(d)+418.08, 相关系数为0.995 2,而且同一粒径的CdTe量子点,共振散射强度与CdTe量子点的浓度也存在良好的线性关系,粒径为3.8 nm的CdTe量子点在波长597 nm处的线性范围,回归方程,相关系数分别为：22.5～180.0 μmol·L-1;I_{597 nm}=0.572 1c+5.884,0.997 5。共振散射光谱法作为检测CdTe量子点粒径的一种新方法,具有简便快速及较好的应用价值。     

水溶性CdTe:Mn量子点的双光子吸收

采用激发波长800 nm、脉宽50 fs、重复频率1 kHz的Ti:sapphire放大飞秒激光器作为激发光源,利用开孔Z扫描技术研究了不同粒径的CdTe:Mn量子点的非线性吸收性质。理论计算结果表明,同一生长时间CdTe:Mn量子点的双光子吸收系数是CdTe量子点的1.1倍,其双光子吸收系数随量子点尺寸的减小而增大,这是由于CdTe:Mn量子点非线性吸收属于反饱和吸收,掺杂了Mn元素,减小了表面缺陷浓度,表明掺杂量子点具有很好的双光子吸收现象。     

Influence of the thickness of CdTe and ZnTe layers on cathodoluminescence spectra of strained CdTe/ZnTe superlattices with quantum-dot layers

The influence of the thickness of ZnTe barrier layers on the cathodoluminescence spectra of strained CdTe/ZnTe superlattices containing layers of quantum dots with an average lateral size of approximately 3 nm has been investigated. In samples with thick barrier layers (30, 15 nm), the cathodoluminescence spectra of quantum dots exhibit one band with a maximum at E = 2.03 eV. It has been revealed that, at a barrier layer thickness of ∼3 nm, the luminescence band is split. However, at a ZnTe layer thickness of 1.5 nm, the luminescence spectrum also contains one band. The experimental results have been interpreted with allowance made for the influence of elastic biaxial strains on the energy states of light and heavy holes in the CdTe and ZnTe layers. For the CdTe/ZnTe heterostructure with quantum dots in which the thickness of the deposited CdTe layer is 1.5 monolayers and the thickness of the barrier layer is 100 monolayers, the cathodoluminescence spectrum contains 2LO-phonon replicas. This effect has been explained by the resonance between two-phonon LO states and the difference between the energy states in the electronic spectrum of wetting layer fragments.     

Surface enhanced Raman scattering by CdS quantum dots

Surface enhanced Raman scattering is studied in nanostructures with CdS quantum dots formed using the Langmuir-Blodgett technology. Features due to quantum dot longitudinal optical phonons are observed in the Raman spectra of both free CdS quantum dots and such dots distributed in an organic matrix. The surface enhanced Raman scattering by nanostructures with CdS quantum dots covered by an Ag cluster film is observed experimentally. Applying Ag clusters onto the nanostructure surfaces results in a sharp (40-fold) increase in the intensity of Raman scattering by optical phonons in the quantum dots. It is shown that the dependence of surface enhanced Raman scattering on the excitation energy is resonant with a maximum at the energy corresponding to the maximum absorption coefficient of Ag clusters.     

GaN基量子点结构的喇曼特征

采用量子力学的微扰理论，对GaN基量子点结构的喇曼频移进行分析。在喇曼实验中，观察InGaN／GaN量子点结构的E2和A1(LO)的模式，并发现实验中样品的喇曼频移与GaN的体材料相比，有着明显的红移。     

Electrical and optical properties of colloidal semiconductor nanocrystals in aqueous environments

Microscale and larger semiconductor crystals have electronic and optical properties that depend on their bulk band structures. When these crystals are reduced into the nanoscale, they enter a new regime in which the electrical and optical properties are no longer influenced solely by their bulk band structures, but are influenced by the crystallite size and shape. In this paper, dimensional confinement and proximity phenomena are examined for colloidal semiconductor nanocrystals in several cases of practical importance. Specifically, we determine the effective binding potentials of selected quantum dots in aqueous environments in various colloidal semiconductor nanocrystals and correlate them with experimentally obtained absorption spectra. We also study fluorescence resonance energy transfer (FRET) between semiconductor crystals connected by short peptide chains as well as the shift in photoluminescence spectra of CdTe nanowires made from a chain of CdTe quantum dots.     

水溶性量子点荧光探针用于帕珠沙星的含量测定

文章采用荧光光谱和紫外光谱研究了CdTe量子点(CdTe QDs)与广谱抗菌药物帕珠沙星的相互作用。结果表明,随着帕珠沙星浓度的增加,CdTe QDs荧光强度有规律的降低,但通过透射电镜图对QDs及加入帕珠沙星后的QDs进行比较,发现QDs仍然均一单分散,表明反应的作用机理可能是帕珠沙星促使QDs表面键合的有机分子发生变化,在Cd空位表现出的表面缺损上形成了碲氧复合物,致使荧光猝灭。因此该反应可作为一种新颖的快速检测帕珠沙星含量的方法。在一定条件下,帕珠沙星溶液的浓度与量子点荧光强度成线性关系,线性范围为10.0～850 μg·mL-1,相关系数r为0.995 4,检测限(S/N=3)为3.254×10-3 μg·mL-1。药物对量子点的猝灭常数为2.188×104 L·mol-1。应用到冻干粉针剂和氯化钠注射液中帕珠沙星的含量测定,所得结果与标示量一致。该方法较常用检测方法具有简便、快捷、灵敏、线性范围宽的优点,并有望进一步开展发药物体内显像及作用机理的研究。     

Optical vibration modes in (Cd,Pb, Zn)S quantum dots in the Langmuir-Blodgett matrix

The structures with CdS, PbS, and ZnS quantum dots produced using the Langmuir-Blodgett method are investigated by infrared (IR) spectroscopy, Raman scattering, and ultraviolet (UV) spectroscopy. The quantum dot size estimated from the UV spectra and high-resolution transmission electron microscopy (HRTEM) falls in the range 2–6 nm. The longitudinal optical (LO) phonons localized in quantum dots and the surface optical vibration modes are revealed in the IR reflection and Raman scattering spectra of the structures under investigation. The frequencies of the surface optical modes are adequately described with allowance made for the effect of localizing optical phonons in the quantum dots.     

IR-active vibrational modes of CdTe and CdSe colloidal quantum dots and CdTe/CdSe core/shell nanoparticles and coupling effects

The frequencies of the vibrational modes of CdTe and CdSe quantum dots and CdTe/CdSe core/shell nanoparticles prepared by the colloid chemistry method are determined using IR transmission and IR reflection spectroscopy. The experimental IR transmission spectrum of CdTe and CdSe nanocrystals exhibits a broad minimum located between the frequencies of the transverse optical (TO) and longitudinal optical (LO) phonons of bulk CdTe and CdSe crystals. The frequencies of the modes for ensembles of CdTe and CdSe quantum dots are considerably shifted toward lower frequencies as compared to those calculated for single quantum dots. This is explained by the dipole-dipole interaction between quantum dots. The frequencies of modes for the structures with core/shell nanoparticles differ little from the calculated frequencies. This suggests a weakening of the interaction in these structures due to the enhancement of dielectric screening.     

不同巯基试剂修饰的CdTe量子点与BSA相互作用研究

修饰试剂对量子点的合成、性质具有重要的影响,但目前有关修饰试剂对量子点与蛋白质间相互作用的影响尚不清楚。采用紫外-可见吸收光谱、荧光光谱及红外光谱研究了3种巯基化合物(巯基乙酸,TGA;L-半胱氨酸,L-Cys;还原型谷胱甘肽,GSH)修饰的CdTe量子点与牛血清白蛋白(BSA)的相互作用。通过Stern-Volmer方程对数据进行了分析,得到了不同CdTe量子点与BSA相互作用过程的ΔHθ,ΔGθ和ΔSθ,并比较了CdTe量子点的不同修饰剂对BSA荧光猝灭的影响。研究结果表明,3种修饰试剂包覆的CdTe量子点与BSA的相互作用均为静态猝灭过程,猝灭常数KSV(L-Cys)KSV(TGA)≈KSV(GSH);TGA和L-Cys修饰的CdTe量子点与BSA的结合力主要为疏水作用力,而GSH修饰的量子点与其结合力既有氢键作用力又有疏水作用力;这些结果说明量子点与BSA的作用过程与量子点表面修饰试剂的类型有关。     

Incorporating fluorescent quantum dots into water-soluble polymer

The fluorescent quantum dot-polymer composites were fabricated by incorporating thioglycolic acid capped CdTe quantum dots into polyacrylamide via cross-linking agents. The CdTe-polyacrylamide composites were characterized by fluorescence spectrophotometer and fluorescence microscope. The result shows that the quantum dot-polymer composites show strong photoluminescence in aqueous solution. The photoluminescence spectrum of quantum dot-polymer composites exhibits a slight blue shift compared to that of initial CdTe quantum dots. The slight shift might be attributed to the covalently bonding between the carboxyl groups of thiolglycolic acid capped on CdTe quantum dots and the amide groups of the polyacrylamide chains.     

Optical investigation of CdS quantum dots in Langmuir–Blodgett films

Structures with CdS quantum dots produced by the Langmuir–Blodgett (LB) technique were investigated by Raman, IR, and UV spectroscopies. The confinement effect of longitudinal optical (LO) phonons in CdS quantum dots was investigated by Raman spectroscopy. Surface vibrational modes of CdS quantum dots were observed in IR spectra. It was shown experimentally that the frequency of the surface vibrational modes depends on the properties of the surrounding media. An average size of CdS quantum dots of about 3–6.4 nm was obtained from the analysis of UV measurements. Received: 1 February 1999 / Accepted: 1 April 1999 / Published online: 19 May 1999     

Raman scattering of Ge dot superlattices

Self-organised Ge dot superlattices grown by molecular beam epitaxy of Ge and Si layers utilizing Stranski-Krastanov growth mode were investigated by Raman spectroscopy. An average size of Ge quantum dots was obtained from transmission electron microscopy measurements. The strain and interdiffusion of Ge and Si atoms in Ge quantum dots were estimated from the analysis of frequency positions of optical phonons observed in the Raman spectra. Raman scattering by folded longitudinal acoustic phonons in the Ge dot superlattices was observed and explained using of elastic continuum theory. Received 25 January 2000     

Interface phonons in semiconductor nanostructures with quantum dots

The vibrational spectra of structures with InAs quantum dots in an AlGaAs matrix and AlAs quantum dots in an InAs matrix are investigated experimentally and theoretically. The Raman spectra exhibit features that correspond to transverse-optical (TO), longitudinal-optical (LO), and interface phonons. The frequencies of interface phonons in InAs and AlAs quantum dots and in an AlGaAs matrix with various concentrations of aluminum are calculated with the use of experimental values of transverse-and longitudinal-optical phonons in the approximation of a dielectric continuum. It is shown that the model of a dielectric continuum adequately describes the behavior of interface phonons in structures with quantum dots under the assumption that the quantum dots are spheroidal.     

Competition between strain and confinement effects on the crystalline quality of InAs/GaAs (001) quantum dots probed by Raman spectroscopy

Self‐organized quantum dots (SOQDs) of InAs/GaAs (001) prepared at low growth temperatures have been carried out by Raman spectroscopy. The structural study performed on these samples using atomic force microscopy showed the presence of two families of quantum dots, and these results were confirmed by analysis of Raman spectra. The low temperature growth leads to smaller dots with nonuniform sizes. The disagreement between the lattice parameters violated the selection rules, and all Raman modes could be observed. SOQDs Raman spectrum shows contribution from the GaAs substrate, the wetting layer, InAs quantum dots and InGaAs alloys at InAs/GaAs interface. A spatial correlation model including the different vibration modes was used to adjust the experimental result. A good agreement of theoretical and experimental results was obtained. Copyright © 2012 John Wiley & Sons, Ltd.