8
RESEARCH PAPER Phase transition in a single VO 2 nano-crystal: potential femtosecond tunable opto-electronic nano-gating M. Maaza A. Simo B. M. Itani J. B. Kana Kana S. El Harthi K. Bouziane M. L. Saboungi T. B. Doyle I. Luk’yanchuk Received: 10 October 2013 / Accepted: 1 April 2014 / Published online: 1 May 2014 Ó Springer Science+Business Media Dordrecht 2014 Abstract The first-order semiconductor–metal Mott transition in single nano-crystal of VO 2 has been observed using scanning tunneling spectroscopy. The variation of the band gap E g with an external thermal stimulus on a single VO 2 nano-crystal in the temper- ature range of 293.5–361.0 K is reported for the first time. The corresponding tuneable IV characteristics versus temperature could be applied in thermally or optically tunable electronic nano-gating in the femto- second regime in view of the ultrafast dynamic in VO 2 . Keywords VO 2 Á Mott transition Á Single nano-crystal Á Band gap Á Scanning tunneling spectroscopy Á Opto-electronic nano-gating Á Instrumentation Introduction Vanadium dioxide (VO 2 ) manifests an ultrafast fem- tosecond first-order semiconductor/metal (SM) phase transition at the vicinity of T C * 340.8 K. Below T C , the band gap E g is approximately 0.70 eV. Above T C , it closes (E g = 0 eV) inducing a considerable reduction in the electrical resistivity by several orders of magnitude ‘‘larger than 10 7 for the bulk material’’ (Morin 1959; Mott 1968; Zylberstejn and Mott 1975; Adler 1966; Goodenough 1971). This electronic phase transition is related to a reversible sharp crystallo- graphic modification: monoclinic and tetragonal below and above T C (Felde et al. 1997; Eyert 2002). The tetragonal rutile (P 42 /mnm) structure has chains of M. Maaza (&) Á A. Simo Á B. M. Itani Á J. B. Kana Kana Á S. El Harthi Á K. Bouziane Á M. L. Saboungi Á T. B. Doyle Á I. Luk’yanchuk College for Graduate Studies, University of South Africa, Muckleneuk Ridge, P O Box 392, Pretoria, South Africa e-mail: [email protected] M. Maaza Á A. Simo Á B. M. Itani Á S. El Harthi Á K. Bouziane Á M. L. Saboungi Á T. B. Doyle Á I. Luk’yanchuk Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation of South Africa, 1 Old Faure Road, Somerset West, Western Cape 7129, South Africa J. B. Kana Kana Department of Materials Science & Engineering, University of Arizona, Tucson, AZ 85721, USA S. El Harthi Department of Physics, College of Sciences, Sultan Qaboos University, Muscat Masqat, Oman M. L. Saboungi Centre de Recherche sur la Matiere Ultradivisee, CNRS, Orle ´ans, France T. B. Doyle School of Chemistry and Physics, University of KwaZulu- Natal, Durban 4001, South Africa I. Luk’yanchuk LPMC, Universite de Picardie Jules Verne, 33, rue St Leu, 80039 Amiens, France 123 J Nanopart Res (2014) 16:2397 DOI 10.1007/s11051-014-2397-z

Phase transition in a single VO2 nano-crystal: potential femtosecond tunable opto-electronic nano-gating

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Page 1: Phase transition in a single VO2 nano-crystal: potential femtosecond tunable opto-electronic nano-gating

RESEARCH PAPER

Phase transition in a single VO2 nano-crystal: potentialfemtosecond tunable opto-electronic nano-gating

M. Maaza • A. Simo • B. M. Itani •

J. B. Kana Kana • S. El Harthi • K. Bouziane •

M. L. Saboungi • T. B. Doyle • I. Luk’yanchuk

Received: 10 October 2013 / Accepted: 1 April 2014 / Published online: 1 May 2014

� Springer Science+Business Media Dordrecht 2014

Abstract The first-order semiconductor–metal Mott

transition in single nano-crystal of VO2 has been

observed using scanning tunneling spectroscopy. The

variation of the band gap Eg with an external thermal

stimulus on a single VO2 nano-crystal in the temper-

ature range of 293.5–361.0 K is reported for the first

time. The corresponding tuneable I–V characteristics

versus temperature could be applied in thermally or

optically tunable electronic nano-gating in the femto-

second regime in view of the ultrafast dynamic in VO2.

Keywords VO2 � Mott transition � Single

nano-crystal � Band gap � Scanning tunneling

spectroscopy � Opto-electronic nano-gating �Instrumentation

Introduction

Vanadium dioxide (VO2) manifests an ultrafast fem-

tosecond first-order semiconductor/metal (SM) phase

transition at the vicinity of TC * 340.8 K. Below TC,

the band gap Eg is approximately 0.70 eV. Above TC, it

closes (Eg = 0 eV) inducing a considerable reduction

in the electrical resistivity by several orders of

magnitude ‘‘larger than 107 for the bulk material’’

(Morin 1959; Mott 1968; Zylberstejn and Mott 1975;

Adler 1966; Goodenough 1971). This electronic phase

transition is related to a reversible sharp crystallo-

graphic modification: monoclinic and tetragonal below

and above TC (Felde et al. 1997; Eyert 2002). The

tetragonal rutile (P42/mnm) structure has chains of

M. Maaza (&) � A. Simo � B. M. Itani �J. B. Kana Kana � S. El Harthi � K. Bouziane �M. L. Saboungi � T. B. Doyle � I. Luk’yanchuk

College for Graduate Studies, University of South Africa,

Muckleneuk Ridge, P O Box 392, Pretoria, South Africa

e-mail: [email protected]

M. Maaza � A. Simo � B. M. Itani � S. El Harthi �K. Bouziane � M. L. Saboungi � T. B. Doyle �I. Luk’yanchuk

Nanosciences African Network (NANOAFNET),

iThemba LABS-National Research Foundation of South

Africa, 1 Old Faure Road, Somerset West,

Western Cape 7129, South Africa

J. B. Kana Kana

Department of Materials Science & Engineering,

University of Arizona, Tucson, AZ 85721, USA

S. El Harthi

Department of Physics, College of Sciences, Sultan

Qaboos University, Muscat Masqat, Oman

M. L. Saboungi

Centre de Recherche sur la Matiere Ultradivisee, CNRS,

Orleans, France

T. B. Doyle

School of Chemistry and Physics, University of KwaZulu-

Natal, Durban 4001, South Africa

I. Luk’yanchuk

LPMC, Universite de Picardie Jules Verne, 33, rue St Leu,

80039 Amiens, France

123

J Nanopart Res (2014) 16:2397

DOI 10.1007/s11051-014-2397-z

Page 2: Phase transition in a single VO2 nano-crystal: potential femtosecond tunable opto-electronic nano-gating

edge-shared VO6 octahedra along the c-axis and the V–

V distance along the chains is 0.2851 nm, while in the

monoclinic (P21/c) crystal structure, the dimerized

vanadium atoms have alternate V–V distances of

*0.2619 and *0.312 nm.

From a theoretical point of view, this SM phase

transition has been interpreted initially in terms of

Mott–Hubbard transition as well as electrons trapping

in homopolar bonds (Goodenough 1971; Eyert 2002).

The recent laser ultrafast spectroscopy investigations

shed lighted on the dynamic of such a phase transition

(Cavalleri et al. 2004; Lysenko et al. 2006). Using a

femtosecond laser pump-probe geometry, the relaxa-

tion processes in VO2 indicated that the light-induced

SM phase transition was as fast as the laser pulse

duration itself (*100 fs). As reported in Fig. 1, the

possible mechanism in relation to the phase transition

may be due to changes in the 3d band configuration

associated with the crystal structure variation. The

upper d||, unoccupied in the semiconductor phase is

within the empty broad p*-band, while strongly

hybridized with oxygen 2p-orbitals and lying above

the Fermi level EF. In the metallic phase, however, all

3d-bands are close to the Fermi level. Upon the laser

pump excitation, the main transitions are from the

occupied d||-valence band to the unoccupied d||–p*

mixed conduction band followed by resonant transi-

tions to unoccupied excited states of the metallic phase.

As a result, the screening of the charge transfer by the

conduction electrons in the metallic phase takes place

by ultrafast laser excitation. The complementary recent

work of Lysenko et al. (2006) indicated that upon an

ultrafast laser excitation, an instantaneous response in

the transient reflectivity and transmission is induced

followed by a relatively longer relaxation process. The

observed phase transition has been associated with the

optical interband transition in VO2. As a consequence

of such a femtosecond reversible MS phase transition,

the optical dielectric constant and thus the refractive

index of VO2 exhibit a temperature modulation which

translates in a large reversible optical modulation in the

infrared spectral region specifically. This singular and

unique property of VO2 makes it a candidate of choice

for smart windows applications, thermal sensors,

optical switching devices, field effect transistors and

electro-optical gates as well as ultrafast tunable nano-

plasmonics, and optical limiting nano-scaled opto-elec-

tronic devices among others (Fan et al. 1977; Gal’perin

et al. 1998; Klimov et al. 2002; Rivera et al. 2009; Maaza

et al. 2000, 2005a, b, 2012; Chen et al. 2004; Balberg and

Trokman 1975; Sella et al. 1998; Wang et al. 2005a, b;

Tsai et al. 2006; Bouchiat et al. 2009).

While extensive studies were conducted on VO2 in

terms of synthesis and investigations of its physical

properties as well as its potential opto-electronic

technological applications, very limited studies were

performed on the variation of its electronic bandgap

Eg(T) in its nano-structured form (Yin et al. 2011;

Adler et al. 1967; Neuman et al. 1964; Kosuge 1967;

Appel 1968; Bongers 1965; Koide and Takei 1967;

Goodenough 1960, 1965; Ladd and Paul 1969; Adler

1966; Sasaki and Watanabe 1964; Zylberstejn and

Mott 1975). For our best knowledge and relatively to

the published literature including the recent work of

Yin et al. (2011), this contribution reports the first

experimental results of the thermal variation of the

Eg(T) of a single VO2 nano-crystal. Likewise, it

demonstrates the possibility to engineer ultrafast and

tunable VO2-based opto-electronic nano-gates.

Experiments and discussion

To perform the intended transport investigations in

single nano-crystals of VO2, it is necessary to engineer

Fig. 1 Reversible opening/closing of the VO2 bandgap versus

external thermal stimuli such as temperature according to Adler

et al. (1967). The phase transition involves mainly d|| and p*

electronic orbitals

2397 Page 2 of 8 J Nanopart Res (2014) 16:2397

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crystalline VO2 films below the coalescence threshold

i.e., consisting of crystallographically textured isolated

nano-particles. Hence pulsed laser deposition was

considered as the technique of choice (Maaza et al.

2000). The target was a pressed powder pellet

(*15 mm in Ø and *2-mm thick) of pure VO2

(Johnson–Matthey 99.99 %). Laser ablation was carried

out in vacuum at a base pressure of*7.5 9 10-8 mbar

with the target rotating at *225 rpm under the XeCl

excimer fundamental laser radiation. F:SnO2-coated

float-glass substrates (5 9 5 9 1 mm3) were used as

transparent electrodes. The coated substrates were

rotated at the same speed as the target, while heated to

a temperature of *674 K. The laser ablation fluence

was adjusted using a quartz lens. The depositions were

carried out at a repetition rate of *10 Hz for a total

number of cycles ranging from 300 to 6,000 in the

fluence range of 0.7–3.2 J/cm2 (Maaza et al. 2000,

2012). The optimal fluence of*1.32 J/cm2 allows high

quality stoichiometric single-phase thermochromic

VO2 nano-crystals. Using the optimal conditions, five

samples of VO2 thin films on F:SnO substrates with

thicknesses of *27 ± 5 nm, 64, 151, 409, and

1,218 ± 2 nm were prepared. This thickness range

which was based on a previous study was considered to

obtain discontinuous and continuous films below and

above the percolation threshold of the VO2 nano-

crystals.

Then, VO2 films surface topology was investigated

using atomic force microscopy (AFM) in the contact

mode. With the exception of the thinnest film

(*27 ± 5 nm in thickness), the other samples (64,

151, 409, and 1,218 ± 2 nm in thickness) showed

almost identical surface morphology as reported in

Fig. 2a which indicates a continuous network of quasi-

spherical nano-crystals with a polydisperse diameter

distribution. Figure 2b reports the surface morphology

of the thinnest VO2 film (*27 ± 5 nm). It indicates a

system of quasi-isolated nano-crystals with an obloid-

like shape anisotropy. The statistical scans provide

average dimensions of *469 ± 12 9 103 ± 9 9

29 ± 7 nm. More accurately, the average height value

of *29 ± 7 nm was obtained from additional statis-

tical measurements over about 30 horizontally posi-

tioned VO2 nano-crystals. One could notice that it is

comparable to the average thickness of the thinnest

film itself (i.e., 27 ± 5 nm), and hence could be

considered as a discontinuous film of single grains.

This thin film sample will be used for the measurement

of the temperature dependence of the I–V characteris-

tics of individual/single VO2 nano-crystals, and hence

the change in the band gap Eg(T) through the SM

transition is as described below.

Fig. 2 AFM surface topography of the thickest (1,218 ± 2 nm,

continuous film) and thinnest (the *27 ± 5 nm, discontinuous,

below percolation threshold) VO2 films in tapping mode

J Nanopart Res (2014) 16:2397 Page 3 of 8 2397

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Page 4: Phase transition in a single VO2 nano-crystal: potential femtosecond tunable opto-electronic nano-gating

To make certain that all films consist of pure VO2,

X-rays diffraction (XRD) and resistivity (R(T)) inves-

tigations were performed on all films. Figure 3a, b

reports the typical XRD and R(T) profiles of the

thickest VO2 films (in this case VO2 film with thickness

of *1,218 ± 2 nm), respectively. These results are

representative of all five samples including the thinnest

discontinuous film. For this latter (i.e., 27 ± 5 nm), as

it was expected, the XRD Bragg peaks and the

resistance/cm2 were broader and larger, respectively.

From the XRD spectra, the room temperature mono-

clinic M1 VO2 phase is confirmed via the presence of

the distinctive (110) Bragg diffraction peak located at

approximately 27.8� corresponding to an average

lattice parameter values of a & 0.5791 nm,

b & 0.4531 nm, and c & 0.5486 nm. From the

R(T) transport perspective, the two orders of magnitude

decrease in the electrical resistance at T & 335 K

(from about 5 9 105 to 7 9 103 X) indicates a (but

incomplete) semiconductor (M1 VO2 phase) to metal

(tetragonal rutile-type structure) phase transition.

Likewise, the stoichiometry of the VO2 films was

further investigated by means of Rutherford Backscat-

tering experiments which gave a ration of vanadium/

oxygen in the range 0.48–0.51 range (±5 %) which

concurs with the stoichiometric value of 0.5.

Scanning tunneling spectroscopy (STS) measure-

ments were performed on the various VO2 film

samples using a Brucker unit equipped with an in-

situ heating stage in the range of 298–573 K with an

accuracy of 0.1 K. The experimental set-up is illus-

trated in Fig. 4 for the thinnest discontinuous film.

STS scans over a representative area of about 1 mm 9

1 mm at temperatures below and above the SM

transition temperature Tc, more precisely at about

338.7 and 361.0 K, respectively, for the thickest VO2

film (1,218 ± 2 nm) are shown in Fig. 5a, b, respec-

tively. Spatial drift due to thermal effects has been

eliminated so that the two figures report the same area/

group of nano-crystals. The surface density of states

(SDS) of Fig. 5 and their spatial distribution were

studied by measuring the I–V characteristics in the

voltage interval from -1 to 1 V by single-point STS

averaging over *15 times of I–V scans in each spot

and grid measurements. The grid for which the

measurements were carried out consisted of a matrix

of 445 9 445 pixels (surface 1.5 9 1.5 nm2). The I–

V curves were taken at each pixel with a bias voltage of

*0.207 V. The results from this time intensive

measurements are illustrated in Fig. 5. Areas where

the occupied electronic states are close to the Fermi

energy, in particular, areas in the metallic state, have

higher tunneling currents and show as dark regions

(gray to black color).Predominantly insulating to a

metallic surface is clearly seen in the evolution of the

images of Fig. 5 as observed by Qazilbash et al. (2007)

by SNOM on thick VO2 films. The white areas

correspond to the semiconducting phases, while the

gray–black regions match up with the metallic VO2

phases. As it can be observed, the cover surface of the

gray–black regions is somehow equivalent to the

white ones at 338.7 K just at the vicinity of TC. This

seems indicating that the Mott phase transition, yet

incomplete, takes place at slightly lower temperatures

20 30 40 50 60 700.0

2.0k

4.0k

6.0k

8.0k

10.0k (33

4)

(60

0)

(52

2)

(43

2)

(22

4)

(51

0)

(42

1)

(42

0)

(00

4)

(41

1)

(40

0)

(31

1)

(31

0)

(22

0)

(10

2)

(21

1)

(11

0)

Inte

nsi

ty (

cps)

2Θ (Deg)

+++: VO2(A)

(a)

(b)

Fig. 3 Typical a XRD and b transport R(T) profiles of the

pulsed laser deposited VO2 thin films onto F:SnO2-coated float-

glass substrate

2397 Page 4 of 8 J Nanopart Res (2014) 16:2397

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Page 5: Phase transition in a single VO2 nano-crystal: potential femtosecond tunable opto-electronic nano-gating

than the bulk value of TC * 341.5 K. Above TC i.e.,

361 K, while almost all of the scanned surfaces are

conducting (gray–black regions), one could notice that

there are still semiconducting domains (white

regions). This could be due either to the amorphous

nature of the surface layer surrounding some of the

VO2 nano-crystals or their oxygen sub-stoichiometry

or to their surface strain/stress. In all three cases, this

surface layer would act as a resistive component and

hence would affect the effective value of the bandgap

Eg(T) of the single VO2 nano-crystals. As it was

evidenced by Wei et al. (2009), the surface strain

phenomenon affects significantly the metal–semicon-

ductor phase transition of VO2. Theoretically, this was

sustained previously by Luk’yanchuk et al. (2009),

while investigating the origin of ferroelastic domains

in free-standing single crystal ferroelectric films. The

near field scanning microwave microscopy investiga-

tions in strained quasi 2-D VO2 nano-platelets by

Tselev et al. (2010a, b) have evidenced the interplay

between ferroelasticity and the SM phase transition as

well as the strain-induced transition between the

insulating M1 and M2 and metallic R phases of VO2.

More precisely, it was demonstrated that the compe-

tition between several phases is purely driven by the

lattice symmetry. Considering the XRD results of

Fig. 3a, it is worth to point to the possibility of co-

existence of both M1 and M2 VO2 phases with the M1

and M2 nano-crystals being crystalline and amor-

phous, respectively. If so, the semiconducting (white)

domains observed at 361 K could be due to the M2

VO2 nano-scaled domains that would not transit

electronically.

In terms of bandgap Eg of a single VO2 (M1) nano-

crystal and its temperature dependence, which is the

cornerstone of this communication, quantitative

assessments by STS have been conducted via I–

V single point measurements in the discontinuous i.e.,

thinnest VO2 film on single isolated nano-crystals of

Fig. 4 The STS configuration to be used to conduct the single

particle I–V measurements on single VO2 nano-crystals in

granular film far from the percolation threshold

Fig. 5 STS of the percolated VO2 film (here the thickest film,

1,218 ± 2 nm in thickness): a just below TC (338.5 K) and

b above TC (358.5 K)

J Nanopart Res (2014) 16:2397 Page 5 of 8 2397

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Page 6: Phase transition in a single VO2 nano-crystal: potential femtosecond tunable opto-electronic nano-gating

VO2. The bandgap Eg is determined from the exten-

sion of the flat section at the center of the dI/dV STS

characteristics, while the spatial distribution of band-

gaps can be visualized by setting a threshold tunneling

current at a set voltage in the dI/dV curves from the

grid measurement. One should mention that Eg values

determined from the flat section are systematically

smaller by 0.1–0.3 eV than if one uses the inflection

point method (Yin et al. 2011). The tunneling thresh-

old current at a set voltage is linked to Eg, which has

been confirmed in a large number of I–V characteristics

of several semiconductors. Figure 6 reports the mea-

sured I–V characteristics for a single isolated single

nano-crystal of VO2 in the thinnest discontinuous film

of 27 ± 5 nm as shown in the inset figure. The I–

V profiles of Fig. 6 have been measured at 298.5,

323.5, 338.5, and 358.5 K. The used tunneling current

was in the range of 0–15 nA for an applied DC voltage

varying from 0 to 2.5 V. At T = 358.5 K i.e., above

Tc, the I–V characteristic is approximately linear, i.e.,

ohmic, pointing to a metallic behavior. At T \ Tc

(298.5, 323.5, and 338.5), the I–V characteristics

exhibit a semiconductor behavior of the investigated

isolated nano-crystal with the width of the ‘‘plateau’’

region being approximately equal to 2 9 Eg(T). As it

can be noticed, width of the plateau and the band gap

Eg(T) vary rapidly at the vicinity of TC. This temper-

ature variation of Eg(T), derived from the standard

derivative dI/dV for various temperatures for the

thinnest discontinuous film, is distinctly illustrated in

Fig. 7. Eg(T) temperature evolution can be splitted in 3

different regions. In region I, Eg(T) decreases almost

linearly, while in region III, its evolution in nearly

inverse exponential with temperature reaches zero

value close to 358.5 K. In region II (temperature range

of 330–340 K), i.e., in the vicinity of TC, it decays

sharply. This trend of Eg(T) in region II could certainly

be correlated to the singular 1st order phase transition

of VO2. However, one should point out 2 major

concerns: (i) the value of Eg(T) in the room temper-

ature range and (ii) the observed temperature variation

of Eg(T) itself. Concerning the values of Eg(T) within

the room temperature range, even with the experi-

mental bar error of ±10 %, the experimentally derived

values of Eg at 298.5 K and at 323.5 K are about

*0.93 and *0.81 eV, respectively, i.e., quite larger

than the bulk value of about 0.7 eV. This difference

could be due to the STS approach itself [23] or/and

either to 3 other major causes. As it was mentioned

previously, the bandgap value Eg deduced from the I–

V flat section is analytically different by 0.1–0.3 eV

than if one uses the inflection point method. This first

source of error has been minimized as the deduced

Eg(T) values were averaged using in fact both I–V and

dI/dV derived values. The 3 additional causes are as

follows: (i) the amorphous nature of the surface layer

surrounding the investigated VO2 single nano-crys-

tals, (ii) its oxygen sub-stoichiometry due to surface

effects such as breakdown of the 3-D symmetry and

atomic coordination, and (iii) its oxygen sub-stoichi-

ometry due to their the strain/stress. In all three cases,

this surface layer would act as a resistive component

and hence would affect the effective value of the

bandgap of the single VO2 nano-crystals. This surface

layer seems to exist in view of the inset zoom of the I–

V curve at 358.5 K. Indeed, the zoom shows clearly

that the I–V evolution is not a complete ohmic type. In

addition, in regard to the recent studies on VO2, the

surface strain hypothesis should be considered too. As

it was evidenced by Wei et al. (2009), the surface

Fig. 6 a Typical I–V curves on a single VO2 nano-crystal in the

non-percolated/discontinuous film at various temperatures:

298.5, 323.5, 338.5, and 358.5 K, b the isolated nano-crystal

on which the measurements were carried out

2397 Page 6 of 8 J Nanopart Res (2014) 16:2397

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Page 7: Phase transition in a single VO2 nano-crystal: potential femtosecond tunable opto-electronic nano-gating

strain phenomenon affects significantly the metal–

semiconductor phase transition of VO2. This was

sustained theoretically by Luk’yanchuk et al. (2009)

while investigating the origin of ferroelastic domains

in free-standing single crystal ferroelectric films. The

near field scanning microwave microscopy investiga-

tions in strained quasi 2-D VO2 nano-platelets by

Tselev et al. (2010a, b) have evidenced the interplay

between ferroelasticity and the metal–semiconductor

phase transition symmetry relationship as well as the

strain-induced transition between the semiconducting

M1 and M2 and metallic R phases of VO2.

From technological applications point of view,

Fig. 6 supports confidently the possibility of using

such a device for ultrafast tunable opto-electronic

gating as the SM reversible transition of the VO2 nano-

crystals can be induced optically in the femtosecond

regime (Cavalleri et al. 2004; Lysenko et al. 2006;

Maaza et al. 2012). Likewise, such an applied aspect is

being extended to engineer tunable ultrafast devices for

surface-enhanced Raman spectroscopy via bang gap

engineering of core shell oxide/VO2 nano-structures

(Balakumar and Ajay Rakkesh 2013; Durgalakshmi

and Balakumar 2013; Parthiban et al. 2013; Ajay

Rakkesh and Balakumar 2013).

Conclusion

The contribution reported on the thermal variation of

the bandgap Eg(T) of VO2(M1) single nano-crystals

synthesized by pulsed laser deposition in a tempera-

ture range below and above the Mott phase transition

taking place at TC * 340.8 K, more precisely in the

range of 298.5–358.5 K. Such a bandgap Eg(T) vari-

ation was deduced from STS via I–V and dI/dV profiles

on single VO2(M1) crystalline nano-particle in a

discontinuous VO2(M1) thin film below the percola-

tion threshold. The variation of Eg(T) at the vicinity of

TC seems to concur with Adler’s crystalline distortion

model. From the technological application perspec-

tive, this contribution has demonstrated the possibility

to engineer reversible and tunable VO2 nano-scaled

femtosecond opto-electronic gates.

Acknowledgments This research program was generously

supported by grants from the National Research Foundation of

South Africa (NRF), the French Centre National pour la

Recherche Scientifique, iThemba LABS, the UNESCO-

UNISA Africa Chair in Nanosciences & Nanotechnology, the

Organization of Women in Science for the Developing World

(OWSDW) and the Abdus Salam ICTP via the Nanosciences

African Network (NANOAFNET) as well as the African Laser

Centre (ALC) to whom we are grateful.

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