Effect of Pressure on Electrical and optical Properties of Metal Doped TiO 2 Shashi Pandey1 Alok Shukla2 Anurag Tripathi1 1Department of Electrical Engineering IET Lucknow Uttar Pradesh 226021 India

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Effect of Pressure on Electrical and optical Properties of Metal Doped TiO2

Shashi Pandey1, Alok Shukla2, Anurag Tripathi1

1Department of Electrical Engineering IET Lucknow, Uttar Pradesh 226021, India

2Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India

Email: 2512@ietlucknow.ac.in, shukla@iitb.ac.in, anurag.tripathi@ietlucknow.ac.in

Abstract

A comparative study of electrical and optical properties of powder and its corresponding pellets

has been done on 3d-doped TiO2. Ti1-xMxO2 (M= Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) powder

its corresponding pellet with doping concentration x= 0.05 has also been prepared using solid

state route. Optical and electrical measurements have been performed for all prepared samples

and interestingly, it is observed that due to having external pressure (i.e. strain) both the

properties change significantly. A rigorous theoretical calculation has also been carried out to

verify the experimental band gap obtained from optical absorption spectroscopy. In case of

pellet sample band gap decreases as compared to the powder sample due to variation of pressure

inside the structures. Role of doping has also been investigated both in pellet and powder forms

and found that the band gap decreases as the atomic number of dopants increases. A cross-over

behavior seen in pellet sample on doping with Ni, Cu and Zn (i.e. band gap increases with

increase in atomic number of dopant). Electrical resistivity measurements have been carried out

for both pellet and powder samples and it is found that in case of strained samples the value of

resistivity is smaller while in case of strain free sample it is quite large. We believe that the

present study suggests a novel approach for tuning the electrical and optical properties of

semiconducting oxides either from doping or from applied pressure (or strain).

Keywords: Titanium dioxide, Doping, Strain, Optical Band gap, Electrical resistivity

Introduction

Recent developments in the field of wide band gap semiconducting oxide materials like TiO2 has

stimulated tremendous research efforts [1–3] focused on studying the influence of dopants on

their multifunctional properties [4–6]. Despite the intense research in the field of science and

technology of semiconductor devices based on GaAs and related group III-V compounds [7],

there are still material issues for higher temperatures and pressures that remain to be better

understood [8,9]. Effect of pressure and doping in semiconducting oxide-based material has

attracted considerable attention of scientific community in recent years because of numerous

potential in the field of electrical and opto-electronic industry [1,9–11]. In most of the recent

works, the main focus to obtain a sufficient level of photocurrent in the devices[10]. Doping and

pressure can also lead to improved optoelectronic and electrical properties in semiconducting

oxides based devices[10,12,13].

Driven by growing concerns about environmental and energy issues, interest in semiconductor-

based opto-electronic devices has increased considerably over the last few decades [10,11,14,15].

Due to its abundance, non-hazardous nature, and high stability under a variety of conditions, TiO2

is a well-studied material ranging from its synthesis to characterization. Furthermore, numerous

experimental and theoretical studies of its physical and chemical properties have already been

performed [4], [5], [10]. For this reason, in this manuscript, we do not focus much on the structural

analysis, instead try to understand the variation of band gap and electrical properties as a function

of its doping with various 3d metals. The electronic band gap of semiconductors tends to decrease

with the increasing external pressure[16]. This behavior can be better understood by realizing that

overlap between the neighboring atomic orbitals increases with decreasing interatomic distance,

leading to enhanced conductivity and reduced band gap. Therefore, direct modulation of the

interatomic distance by applying high compressive/tensile stress provides a pathway to tuning of

the bandgap[9,17].

The purpose of this article is to study the variation in electrical and optical properties TiO2 as a

function of the type of dopant, doping concentration, and external pressure. For the purpose we

prepare powder and its pellets of Ti1-xMxO2 to experimentally study the variation of electrical and

optical properties. Additionally, we also theoretically support our experimental work by means of

first-principles density-functional theory calculations.

Experimental and Computational Details

We prepared Ti1-xMxO2 pellets by solid state route by mixing the dopant (M = Sc, V, Cr, Mn, Fe,

Co, Ni, Cu, Zn) and TiO2 powder in required and subsequent to that we add isopropyl alcohol

[CH3CH2 (OH) CH3)] and then grind the sample for one hour. After that we add binder to it in

small amount, and grind the sample again for about 15 minutes. Subsequently, we pelletize the

powder using 1-inch diset of applied pressure. Next, the pellet is heat treated at 900 0C for twelve

hours finally resulting in a 1-inch target of Ti1-xMxO2. The band gap of TiO2 and 3d-transition

metal doped TiO2 were determined by using UV–VIS spectroscopy with the wavelength range

300-750 nm. Electrical resistivity measurements were done using the four-probe method and the

experimental set up consists of probe arrangement, sample, constant current generator, oven power

supply and digital panel meter, for measuring the voltage and current. Morphology of pellet and

powder samples has been characterized using scanning electron microscopy (SEM) supra Zeiss

and Carl Zeiss in plan-views arrangement. All first principles calculations[18] were performed

using plane-wave density functional theory (PW-DFT)[19,20] implemented in Quantum Espresso

simulation package [21] with generalized gradient approximation (GGA+U)[22–25]. Calculations

have been performed on pure TiO2 and 3d- doped TiO2, with supercell of 3x2x2. The k-mesh of

size 6 x 6 x 6 in the first Brillouin zone has been used for pure and 5% lattice contracted system.

The self-consistent calculations were considered to be converged, when the total energy of the

system is stable within 10-3mRy, forces per atom declined to less than 0.04 eV/A˚ and the energy

convergence up to 5x10-5 eV[12,26].

Results and Discussion

The effect of pressure on band gap can be understood in quite simple terms. Pressure changes the

lattice parameters and, therefore, the average distance between the ions and the charge carriers.

This modifies the potential felt by the charge carriers due to the ions, resulting in the change in the

band gap. Comparative morphological study on interfacial strain across grain boundaries has been

done on pellet and powder in TiO2. SEM results for pellet and powder samples have been studied

and see the grain contribution to produce interfacial strain across phase coexistence. A

comparative study of SEM images in Figure-1(a) & 1(b) shows that in pellet sample grain

boundaries are closed packed even crystal plane boundaries are well organized with no spacing.

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EffectofPressureonElectricalandopticalPropertiesofMetalDopedTiO2ShashiPandey1,AlokShukla2,AnuragTripathi11DepartmentofElectricalEngineeringIETLucknow,UttarPradesh226021,India2DepartmentofPhysics,IndianInstituteofTechnologyBombay,Powai,Mumbai400076,IndiaEmail:2512@ietlucknow.ac.in,shukla@iitb.ac.in,a...

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Effect of Pressure on Electrical and optical Properties of Metal Doped TiO 2 Shashi Pandey1 Alok Shukla2 Anurag Tripathi1 1Department of Electrical Engineering IET Lucknow Uttar Pradesh 226021 India

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