Protasenko Vladimir, Bacinello Daniel, Kuno Masaru
Department of Chemistry and Biochemistry and Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, USA.
J Phys Chem B. 2006 Dec 21;110(50):25322-31. doi: 10.1021/jp066034w.
Absorption cross-sections and corresponding molar extinction coefficients of solution-based CdSe and CdTe nanowires (NWs) are determined. Chemically grown semiconductor NWs are made via a recently developed solution-liquid-solid (SLS) synthesis, employing low melting Au/Bi bimetallic nanoparticle "catalysts" to induce one-dimensional (1D) growth. Resulting wires are highly crystalline and have diameters between 5 and 12 nm as well as lengths exceeding 10 microm. Narrow diameters, below twice the corresponding bulk exciton Bohr radius of each material, place CdSe and CdTe NWs within their respective intermediate to weak confinement regimes. Supporting this are solution linear absorption spectra of NW ensembles showing blue shifts relative to the bulk band gap as well as structure at higher energies. In the case of CdSe, the wires exhibit band edge emission as well as strong absorption/emission polarization anisotropies at the ensemble and single-wire levels. Analogous photocurrent polarization anisotropies have been measured in recently developed CdSe NW photodetectors. To further support fundamental NW optical/electrical studies as well as to promote their use in device applications, experimental absorption cross-sections are determined using correlated transmission electron microscopy, UV/visible extinction spectroscopy, and inductively coupled plasma atomic emission spectroscopy. Measured CdSe NW cross-sections for 1 microm long wires (diameters, 6-42 nm) range from 6.93 x 10(-13) to 3.91 x 10(-11) cm2 at the band edge (692-715 nm, 1.73-1.79 eV) and between 3.38 x 10(-12) and 5.50 x 10(-11) cm2 at 488 nm (2.54 eV). Similar values are obtained for 1 microm long CdTe NWs (diameters, 7.5-11.5 nm) ranging from 4.32 x 10(-13) to 5.10 x 10(-12) cm2 at the band edge (689-752 nm, 1.65-1.80 eV) and between 1.80 x 10(-12) and 1.99 x 10(-11) cm2 at 2.54 eV. These numbers compare well with previous theoretical estimates of CdSe/CdTe NW cross-sections far to the blue of the band edge, having order of magnitude values of 1.0 x 10(-11) cm2 at 488 nm. In all cases, experimental NW absorption cross-sections are 4-5 orders of magnitude larger than those for corresponding colloidal CdSe and CdTe quantum dots. Even when volume differences are accounted for, band edge NW cross-sections are larger by up to a factor of 8. When considered along with their intrinsic polarization sensitivity, obtained NW cross-sections illustrate fundamental and potentially exploitable differences between 0D and 1D materials.
测定了基于溶液的CdSe和CdTe纳米线（NWs）的吸收截面和相应的摩尔消光系数。通过最近开发的溶液-液-固（SLS）合成法制备了化学生长的半导体纳米线，采用低熔点的Au/Bi双金属纳米颗粒“催化剂”来诱导一维（1D）生长。所得的纳米线具有高度结晶性，直径在5至12纳米之间，长度超过10微米。窄直径（低于每种材料相应体相激子玻尔半径的两倍）使CdSe和CdTe纳米线处于各自的中间至弱限制区域。纳米线集合体的溶液线性吸收光谱显示相对于体相带隙的蓝移以及更高能量处的结构，这支持了上述结论。对于CdSe，纳米线在集合体和单根纳米线水平上均表现出带边发射以及强烈的吸收/发射偏振各向异性。在最近开发的CdSe纳米线光电探测器中也测量到了类似的光电流偏振各向异性。为了进一步支持基础的纳米线光学/电学研究以及促进其在器件应用中的使用，使用相关透射电子显微镜、紫外/可见消光光谱和电感耦合等离子体原子发射光谱来测定实验吸收截面。对于1微米长的纳米线（直径为6 - 42纳米），在带边（692 - 715纳米，1.73 - 1.79电子伏特）处，测得的CdSe纳米线截面范围为6.93×10⁻¹³至3.91×10⁻¹¹平方厘米，在488纳米（2.54电子伏特）处为3.38×10⁻¹²至5.50×10⁻¹¹平方厘米。对于1微米长的CdTe纳米线（直径为7.5 - 11.5纳米），在带边（689 - 752纳米，1.65 - 1.80电子伏特）处，得到类似的值，范围为4.32×10⁻¹³至5.10×10⁻¹²平方厘米，在2.54电子伏特处为1.80×10⁻¹²至1.99×10⁻¹¹平方厘米。这些数值与先前对远在带边蓝光区域的CdSe/CdTe纳米线截面的理论估计值相当，在488纳米处的量级值为1.0×10⁻¹¹平方厘米。在所有情况下，实验测得的纳米线吸收截面比相应的胶体CdSe和CdTe量子点的吸收截面大4 - 5个数量级。即使考虑了体积差异，带边纳米线截面仍大至8倍。结合其固有的偏振敏感性来看，所得到的纳米线截面说明了0D和1D材料之间的基本差异以及潜在的可利用差异。
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