The Shankar Research Group - Excitonics and Nanostructures Laboratory



65. Adl AH, Kar P, Farsinezhad S, Sharma H and Shankar K, Effect of Sol Stabilizer on the Structure and Electronic Properties of Solution-Processed ZnO thin Films, RSC Advances, 5, 87007 - 87018, 2015.

Placeholder image

Abstract: ZnO is an increasingly important wide bandgap semiconductor for optoelectronic applications. Solution processing provides a facile and inexpensive method to form ZnO thin films with high throughput. The sol stabilizer used in the solution processing of ZnO functions variously as a sol homogenizer, chelating agent, wettability improver and capping agent. In spite of its obvious importance in influencing ZnO film properties, a restricted set of short chain alkaline sol stabilizers have been used in prior reports. We examined the effect of six different sol stabilizers, including acidic and longer chain species, along with a recipe without any stabilizer, on the grain size, crystallographic texture, and resistivity of solution processed ZnO films on thermal oxide-coated silicon substrates, and found large variations in the structural and electrical properties as a consequence of the choice of sol stabilizer. We found that ZnO films formed using oleic acid as the sol stabilizer possessed a strong (002) preferred orientation with a Lotgering factor as high as 0.86. The key insight we obtained is that the sol stabilizer strongly influences the film surface area and activation energy barrier for inter-grain transport. We comprehensively studied the steady state and transient behavior of ZnO films deposited using different stabilizers and compared their lifetime and mobility-lifetime products. When exposed to illumination, the conductivity of the deposited films increased by several orders of magnitude. This is attributed to the trapping of the nonequilibrium holes by the surface adsorbed oxide species, which produces equivalent number of excess electrons in the conduction band. Impedance spectroscopy and C–V measurements were performed to calculate the doping of the ZnO thin films. ZnO thin film transistors were also fabricated and the effects of the sol stabilizer on the different parameters of the TFT like mobility and threshold voltage were investigated.

64. Farsinezhad S, Sharma H and Shankar K, Interfacial band alignment for photocatalytic charge separation in TiO2 nanotube arrays coated with CuPt nanoparticles, Physical Chemistry Chemical Physics, 17, 29723-29733A, 2015.

Placeholder image

Abstract: n-Type anatase-phase one-dimensional TiO2 nanostructure arrays coated with nanoparticles of Cu or CuPt have emerged as high performance photocatalysts for both photooxidation and photoreduction. The properties of the catalyst-promoter interface are recognized to be critical to this high performance but are largely unknown. Using X-ray and ultraviolet photoelectron spectroscopies (XPS/UPS), we probed the electronic properties of the CuPt–TiO2 interface in transparent TiO2 nanotube arrays (TTNTAs) coated with photodeposited CuPt nanoparticles (CuPt–TTNTA hybrids) as well as those coated with sputtered CuPt (Sput-CuPt–TTNTA hybrids). XPS and UPS spectra provided the evidence of a Schottky barrier with a band-bending of 0.49–0.67 eV at the CuPt–TiO2 interface in CuPt–TTNTA hybrids due to which photoexcited electrons are expected to be retained in the TiO2 while photoexcited holes will be collected by the CuPt nanoparticles. For Sput-CuPt–TTNTA hybrids, no such band-bending was observed. These results point to the importance of the metal nanoparticle preparation technique on interfacial band-alignments and challenge the conventional understanding of the promoting action of noble metal nanoparticles on TiO2 photocatalysts as sinks for photoexcited electrons.

63. Zarifi MH, Mohammadpour A, Farsinezhad S, Wiltshire BD, Nosrati M, Askar AM, Daneshmand M and Shankar K, TRMC Using Planar Microwave Resonators: Application to the Study of Long-lived Charge Pairs in Photoexcited Titania Nanotube Arrays, Journal of Physical Chemistry C, 119 (25), 14358-14365, 2015.

Placeholder image

Abstract: Steady-state (SRMC) and time-resolved microwave photoconductivity (TRMC) are key techniques used to perform the contact-less determination of carrier density, transport, trapping, and recombination parameters in charge transport materials such as organic semiconductors and dyes, inorganic semiconductors, and metal–insulator composites, which find use in conductive inks, thin film transistors, light-emitting diodes, photocatalysts, and photovoltaics. We present the theory, design, simulation, and fabrication of a planar microwave ring resonator operating at 5.25 GHz with a quality factor of 224, to perform SRMC and TRMC measurements. Our method consists of measuring the resonance frequency (f0) and Q-factor of the microwave resonator with the sample to be probed placed in a defined sensitive region of the resonator where the microwave field is highly concentrated. We also provide proof of concept measurements of the time-resolved microwave photoresponse of anatase-phase TiO2 nanotube array membranes (TNTAMs) using the planar microstrip resonator. An unusual observation was the persistence of charged pair states in TNTAMs for several hours at room temperature under ambient conditions. Fast (120–220 s), slow (1300–2850 s), and very slow (6–26 h) components were extracted from the long-lived photoconductive decays of TNTAMs in response to 365, 250, and 405 nm illumination and assigned to various trap-mediated processes in TiO2 nanotubes.

62. Sharma H, Krabbe JD, Farsinezhad S, van Popta AC, Wakefield NG, Fitzpatrick GA and Shankar K, Mapping Stresses in High Aspect Ratio Polysilicon Electrical Through-
Wafer Interconnects (ETWIs)
, Journal of Micro/Nanolithography, MEMS, and MOEMS (JM3), 14(2), 024001, 2015.

Abstract: Abstract. Electrical through-wafer interconnect technologies such as vertical through-silicon vias (TSVs) are essential in order to maximize performance, optimize usage of wafer real estate, and enable three-dimensional packaging in leading edge electronic and microelectromechanical systems (MEMS) products. Although copper TSVs have the advantage of low resistance, highly doped polysilicon TSVs offer designers a much larger range of processing options due to the compatibility of polysilicon with high temperatures and also with the full range of traditional CMOS processes. Large stresses are associated with both Cu and polysilicon TSVs, and their accurate measurement is critical for determining the keep-out zone (KOZ) of transistors and for optimizing downstream processes to maintain high yield. This report presents the fabrication and stress characterization of 400-μm 400-μm deep, 20-Ω 20-Ω resistance, high aspect ratio (25:1) polysilicon TSVs fabricated by deep reactive ion etching (DRIE) followed by low-pressure chemical vapor deposition (LPCVD) of polysilicon with in-situ boron doping. Micro-Raman imaging of the wafer surface showed a maximum stress of 1.2 GPa occurring at the TSV edge and a KOZ of ∼9∼9 to 11μm11 μm. For polysilicon TSVs, the stress distribution in the TSVs far from the wafer surface(s) was not previously well-understood due to measurement limitations. Raman spectroscopy was able to overcome this limitation; a TSV cross section was examined and stresses as a function of both depth and width of the TSVs were collected and are analyzed herein. An 1100°C postanneal was found to reduce average stresses by 40%.

61. Zarifi MH, Farsinezhad S, Shankar K and Daneshmand M, Liquid Sensing Using Active Feedback Assisted Co-Planar Microwave Resonator, IEEE Microwave and Wireless Components Letters, 25, 621-623, 2015.

Abstract: A novel electromagnetic sensor operating at microwave frequencies with quality factor of 22,000 at 1.4 GHz for real-time sensing of fluid properties is presented. The core of the sensor has a planar microstrip resonator, which is enhanced using an active feedback loop. The resonance frequency and quality factor of the sensor show clear differentiation between analytes composed of common solvents. To evaluate the sensor for water based concentration detection, we have demonstrated that KOH dilutions as low as 0.1 mM are detectable. The proposed sensor has advantages of inexpensiveness and high resolution as well as capability for miniaturization and CMOS compatibility.

60. [Invited Review Article] Piyush Kar, Arash Mohammadpour, Benjamin Daniel Wiltshire, Abdelrahman Askar and Karthik Shankar, Electron Transport, Trapping and Recombination in Anodic TiO2 Nanotube Arrays, Current Nanoscience - Special Issue on Nanoporous Anodic Oxides, 11(5) 593-614, 2015.

Placeholder image

Abstract: Anodically synthesized TiO2 nanotube arrays (TNTAs) constitute an exciting ordered large bandgap semiconductor nanoarchitecture for use as scaffolds and active layers for solutionprocessable devices including but not limited to, optoelectronic sensors, photovoltaics, photodetectors, photocatalysts and photoelectrochemical cells. Charge transport, trapping and recombination are key attributes of the material architecture that significantly influence the properties and performance of the resulting optoelectronic devices, thus motivating this review article. Since nanocrystalline mesoporous TiO2 films (np-TiO2) are actively researched for the same applications, in many cases, TNTAs and np-TiO2 are direct competitors and it is therefore meaningful to compare the optoelectronic properties of the two architectures head-to-head. In addition, there exists a whole host of TNTA-specific applications such as bottom-up fabricated photonic crystals, bulk heterojunction organic solar cells and metallodielectric metamaterials that leverage the ordered channel architecture. Recent studies have established the order of magnitude superior recombination lifetimes in sensitized TNTAs as compared to sensitized np-TiO2 as well as the salutary effect of lower structural disorder in TNTAs resulting in trap-free electron diffusion coefficients approaching those of single crystals and two orders of magnitude larger than np-TiO2. Photoconductivity measurements using bandgap illumination in both single nanotubes and nanotube ensembles have resulted in similar values of the mobility-lifetime product (10-5-10-4 cm2V-1), which are four to six orders of magnitude higher than in nanoparticle electrodes. At the same time, TiO2 nanotubes have a larger trap density and a greater average trap-depth than nanoparticulate Ti2 films, pointing to the importance of synthesis modification to improve material quality and post-synthesis techniques for trap passivation.

59. Piyush Kar , Yun Zhang , Samira Farsinezhad , Arash Mohammadpour , Benjamin Daniel Wiltshire , Himani Sharma and Karthik Shankar, Rutile phase n- and p-type anodic titania nanotube arrays with square-shaped pore morphologies, Chemical Communications, 51, 7816-7819, 2015.

Abstract: Rutile-phase TiO2 nanotube arrays without broken walls were formed by annealing of anodically formed nanotubes in a propane flame at 650 C and in air at 750 C. An unusual morphological transformation was observed from the ellipsoidal pore-shapes of titania nanotubes grown in aqueous electrolyte to a square-shaped pore structure subsequent to the anneals. 750 C annealed nanotubes were found to be lightly p-type, rare in TiO2.

58. Gang He, Benjamin Daniel Wiltshire, Paul Choi, Aliaksandr Savin, Shuai Sun, Arash Mohammadpour, Michael J Ferguson, Robert McDonald, Samira Farsinezhad, Alex Brown, Karthik Shankar and Eric Rivard, Phosphorescence within Benzotellurophenes and Color Tunable Tellurophenes under Ambient Conditions, Chemical Communications, 51, 5444-5447, 2015.

Placeholder image

Abstract: The zirconium-mediated syntheses of pinacolboronate (BPin) appended benzo[b]tellurophenes and two phenyl/BPin substituted tellurophene isomers with different colors of emission have been achieved. These species are new additions to an emerging class of inorganic heterocycles that display visible phosphorescence in the solid state under ambient conditions.

57. Joel Boulet, Arash Mohammadpour and Karthik Shankar, Insights into the Solution Crystallization of Oriented Alq3 and Znq2 Microprisms and Nanorods, Journal of Nanoscience and Nanotechnology, 15, 6680-6689, 2015.

Abstract: Optimized solution-based methods to grow high quality micro- and nanocrystals of organic semiconductors with defined size, shape and orientation are important to a variety of optoelectronic applications. In this context, we report the growth of single crystal micro- and nanostructures of the organic semiconductors Tris(8-hydroxyquinoline)aluminum (Alq3) and bis(8-hydroxyquinoline)zinc (Znq2) terminating in flat crystal planes using a combination of evaporative and antisolvent crystallization. By controlling substrate-specific nucleation and optimizing the conditions of growth, we generate vertically-oriented hexagonal prism arrays of Alq3, and vertical half-disks and sharp-edged rectangular prisms of Znq2. The effect of process variables such as ambient vapour pressure, choice of anti-solvent and temperature on the morphology and crystal habit of the nanostructures were studied and the results of varying them catalogued to gain a better understanding of the mechanism of growth.