Sajid Ullah Khan

Patterning Lead Zirconate Titanate Nanostructures at Sub-200-nm Resolution by Soft Confocal Imprint Lithography and Nanotransfer Molding

Patterned sol-gel-derived lead zirconate titanate (PZT) thin films with lateral resolutions down to 100 nm on silicon are reported. Both an imprint and a transfer-molding method were employed. The formed patterns after annealing were characterized with scanning electron microscopy, atomic force microscopy, and X-ray diffraction. Despite the small dimensions and flexibility of the poly(dimethylsiloxane) (PDMS) stamps used for patterning, the quality of replication was found to be good. The influence of the surface energies of the substrate, PDMS mold, and precursor solution on the quality of pattern replication is discussed. The colloidal structure of the PZT sol-gels from which the patterns were made was studied with small-angle X-ray scattering. The sols were found to be chemically homogeneous down to a length scale of approximately 2 nm and higher, which is sufficient for pattern replication on approximately 100 nm scale.

Cd-doping a facile approach for better thermoelectric transport properties of BiCuSeO oxyselenides

BiCuSeO-based thermoelectric materials have spurred tremendous interest among the thermoelectric community due to their ultra-low thermal conductivity and relatively large Seebeck coefficient (S). In this work, we have reported the effect of Cd-doping at the Bi site, instead of the previously studied Cu site, on the thermoelectric performance of BiCuSeO by modifying the insulating layer. While maintaining good phase purity, Cd was successfully doped at the Bi site as confirmed by X-ray absorption fine structure spectroscopy. The Cd-doping substantially improves the electrical conductivity by a factor of 20 through bond anharmonicity at room temperature while increasing the Cd concentration over 5%. Further, the incorporation of the lighter atom at the Bi site creates phonon scattering centers and results in weak bonding between the layers, resulting in a remarkable perturbation of the local geometric and electronic structure. BiCuSeO with 5% Cd-doping maintains a large S and a high electrical conductivity up to 923 K and exhibits the highest power factor values (600 μW m−1 K−2 at 323 K and 447 μW m−1 K−2 at 923 K) and the largest ZT (0.98 at 923 K). Cd-doping at the Bi site in p-type thermoelectric BiCuSeO was shown to be a very good technique for improving the thermoelectric performance and could be extended to other thermoelectric materials to enhance the efficiency of thermoelectric devices for energy-harvesting.

Enhanced Thermoelectricity in High-Temperature β-Phase Copper(I) Selenides Embedded with Cu2Te Nanoclusters

We report remarkably enhanced thermoelectric performance of Te doped Cu2Se in midtemperature range. Through ball-milling process followed by spark plasma sintering (SPS), nanoscale Cu2Te clusters were embeded in the matrix of Cu2Se, inducing a drastic enhancement of thermoelectric performance by reducing the thermal conductivity without degrading the power factor. A large ZT value of 1.9 was achieved at 873 K for Cu2Se1.9Te0.1, which is about 2 times larger than that of the pure Cu2Se. The nanoscale heat management by Cu2Te nanoclusters in superionic conductors opens up an avenue for thermoelectric materials research.

Pronounced effect of ZnTe nanoinclusions on thermoelectric properties of Cu 2−x Se chalcogenides

Metal chalcogenides especially Cu2−xSe has gained much attention in thermoelectric community due to its complex crystal structure and superionic behavior. Here, we report a facile method to improve the thermoelectric efficiency by introducing ZnTe nanoinclusions into the matrix of Cu2−xSe. As a result, a substantial improvement of 32% in electrical conductivity of Cu2−xSe-ZnTe composite is observed. The increase in electrical conductivity is at the expense of Seebeck coefficient, which slightly decreases the power factor of the composite samples than that of pure Cu2−xSe. Furthermore, the introduction of secondary phase facilitates in declining the total thermal conductivity of Cu2−xSe-ZnTe composite up to 34% by suppressing the lattice thermal contributions. Thus, the moderate power factor and lower thermal conductivity values result in an improved figure of merit (zT) value of ∼0.40 in mid-range temperature (750 K) for Cu2−xSe-ZnTe composite with 10 wt.% of ZnTe, which is about 40% higher than that of its pure counterpart. Hence, it is believed that the incorporation of ZnTe nanoinclusions in the matrix of Cu2−xSe may be an important route to improve the thermoelectric properties of Cu2−xSe based compounds.摘要由于具有复杂的晶体结构和超离子导体行为, 金属硫属化合物特别是Cu2−xSe在热电领域得到了广泛的关注. 本文报道了一种简单易行的提高热电效率的方法:在基体材料Cu2−xSe中添加纳米ZnTe插层, 用来提高Cu2−xSe材料的热电性能. 实验结果表明, Cu2−xSe-ZnTe复合材料的电导率提高了32%, 电导率的增加牺牲了塞贝克系数, 导致复合材料的功率因子稍微低于纯Cu2−xSe基体材料; 第二相的引入抑制了晶格热扩散, 使得Cu2−xSe-ZnTe复合材料的热导率降低了34%. 由此可知, 适中的功率因子和较低的热导率致使含有10 wt.%ZnTe的Cu2−xSe-ZnTe复合材料在中温条件(750 K)下的zT值提高至0.40, 相比于纯Cu2−xSe基体材料该数值提高了40%. 因此, 向Cu2−xSe材料中添加纳米ZnTe插层, 是提高Cu2−xSe基材料热电性能的一个有效途径.

Patterning luminescent nanocrystalline LaPO 4 : Eu and CePO 4 : Tb particles embedded in hybrid organosilica with soft-lithographic techniques

Eu3+-doped LaPO4 and Tb3+-doped CePO4 luminescent nanoparticles embedded in hybrid organosilica were patterned by two soft lithographic techniques. The role of various parameters such as solution chemistry, thermal protocols, and modification of the mold-substrate surface energies related to pattern shape formation and adhesion to the substrates have been studied. The shrinkage of the oxide patterns and shape evolution during the process was also examined. The patterns were characterized with optical and photoluminescence (PL) microscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Compositional analyses were carried out with X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and secondary ion mass spectroscopy (SIMS). The results indicated that the final patterns obtained with these two techniques for the same material have different shapes and adherence to the substrates.

Facile synthesis of gold nanostars over a wide size range and their excellent surface enhanced Raman scattering and fluorescence quenching properties

Surface enhanced Raman scattering (SERS) is an excellent technique for detecting the trace concentration of targets. However, SERS based detection of fluorescent targets is tricky as the fluorescence signal can severely interfere with the SERS signal upon resonance excitation. This can limit the trace detection capability of SERS. Herein, the authors demonstrate that Au nanostars (NSs) are excellent for fluorescence quenching and trace detection by SERS. The NS based SERS substrate was used to measure the R6G concentrations as low as 10 pM upon laser excitation that are in resonance with the fluorescence absorption. The authors used a simple seed-mediated and surfactant assisted method to synthesize AuNSs in a wide size range. The size of the NSs can be tuned from 55u2009nm to about 1u2009μm simply by varying the seed to HAuCl4 ratio in the growth solution.Surface enhanced Raman scattering (SERS) is an excellent technique for detecting the trace concentration of targets. However, SERS based detection of fluorescent targets is tricky as the fluorescence signal can severely interfere with the SERS signal upon resonance excitation. This can limit the trace detection capability of SERS. Herein, the authors demonstrate that Au nanostars (NSs) are excellent for fluorescence quenching and trace detection by SERS. The NS based SERS substrate was used to measure the R6G concentrations as low as 10 pM upon laser excitation that are in resonance with the fluorescence absorption. The authors used a simple seed-mediated and surfactant assisted method to synthesize AuNSs in a wide size range. The size of the NSs can be tuned from 55u2009nm to about 1u2009μm simply by varying the seed to HAuCl4 ratio in the growth solution.

Pronounced effect of ZnTe nanoinclusions on thermoelectric properties of Cu2−xx

Metal chalcogenides especially Cu2−xSe has gained much attention in thermoelectric community due to its complex crystal structure and superionic behavior. Here, we report a facile method to improve the thermoelectric efficiency by introducing ZnTe nanoinclusions into the matrix of Cu2−xSe. As a result, a substantial improvement of 32% in electrical conductivity of Cu2−xSe-ZnTe composite is observed. The increase in electrical conductivity is at the expense of Seebeck coefficient, which slightly decreases the power factor of the composite samples than that of pure Cu2−xSe. Furthermore, the introduction of secondary phase facilitates in declining the total thermal conductivity of Cu2−xSe-ZnTe composite up to 34% by suppressing the lattice thermal contributions. Thus, the moderate power factor and lower thermal conductivity values result in an improved figure of merit (zT) value of ∼0.40 in mid-range temperature (750 K) for Cu2−xSe-ZnTe composite with 10 wt.% of ZnTe, which is about 40% higher than that of its pure counterpart. Hence, it is believed that the incorporation of ZnTe nanoinclusions in the matrix of Cu2−xSe may be an important route to improve the thermoelectric properties of Cu2−xSe based compounds.摘要由于具有复杂的晶体结构和超离子导体行为, 金属硫属化合物特别是Cu2−xSe在热电领域得到了广泛的关注. 本文报道了一种简单易行的提高热电效率的方法:在基体材料Cu2−xSe中添加纳米ZnTe插层, 用来提高Cu2−xSe材料的热电性能. 实验结果表明, Cu2−xSe-ZnTe复合材料的电导率提高了32%, 电导率的增加牺牲了塞贝克系数, 导致复合材料的功率因子稍微低于纯Cu2−xSe基体材料; 第二相的引入抑制了晶格热扩散, 使得Cu2−xSe-ZnTe复合材料的热导率降低了34%. 由此可知, 适中的功率因子和较低的热导率致使含有10 wt.%ZnTe的Cu2−xSe-ZnTe复合材料在中温条件(750 K)下的zT值提高至0.40, 相比于纯Cu2−xSe基体材料该数值提高了40%. 因此, 向Cu2−xSe材料中添加纳米ZnTe插层, 是提高Cu2−xSe基材料热电性能的一个有效途径.