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Emerging photoluminescence from defective

vanadium diselenide nanosheets

AMIR GHOBADI,

1,2,5

TURKAN GAMZE ULUSOY GHOBADI,

ALI KEMAL OKYAY,

2,3

AND EKMEL OZBAY

1,2,3,4,

NANOTAM-Nanotechnology Research Center, Bilkent University, 06800 Ankara, Turkey

Department of Electrical and Electronics Engineering, Bilkent University, 06800 Ankara, Turkey

UNAM-Institute of Materials Science and Nanotechnology, Bilkent University, 06800 Ankara, Turkey

Department of Physics, Bilkent University, 06800 Ankara, Turkey

e-mail: amir@ee.bilkent.edu.tr

*Corresponding author: ozbay@bilkent.edu.tr

Received 12 December 2017; revised 17 January 2018; accepted 21 January 2018; posted 23 January 2018 (Doc. ID 315573);

published 22 March 2018

In this paper, for the first time to our knowledge in the literature, we demonstrate photoluminescence from

two-dimensional (2D) vanadium diselenide (VSe

) nanosheets (NSs). The preparation of these nanostructures

is carried out with a combinational method based on nanosecond pulsed laser ablation (PLA) and chemical

exfoliation. For this aim, VSe

bulk is first ablated into nanoparticles (NPs) inside a water solution. Afterward,

NPs are chemically exfoliated into NSs using lithium intercalation via ultrasonic treatment. Although VSe

is a

semimetal in its bulk form, its nanostructures show photo-responsive behavior, and it turns into a strongly

luminescent material when it is separated into NSs. Based on the obtained results, the surface defects induced

during the PLA process are the origin of this photoluminescence from NSs. Our findings illustrate that this new

material can be a promising semiconductor for photovoltaic and light emitting diode applications.

Chinese Laser Press

OCIS codes: (130.5990) Semiconductors; (160.4670) Optical materials; (250.5230) Photoluminescence.

https://doi.org/10.1364/PRJ.6.000244

1. INTRODUCTION

The discovery of the intriguing electrical and physical proper-

ties of two-dimensional (2D) materials [1–3], such as graphene,

has inspired scientists to synthesize other new types of 2D

materials. Transition metal dichalcogenides (TMDs) are one

kind of these recently explored materials and have been strik-

ingly highlighted in recent years [4–8]. Unlike graphene, which

is inherently a semimetal with zero optical band gap, TMDs

can show diverse bulk properties from insulators (HfS

), semi-

conductors (MoS

, WS

), and semimetals (WTe

, TiSe

), to

true metals (NbS

, VSe

). For this reason, in recent years,

TMDs have been promising building blocks for a wide range

of applications, including photovoltaics [9–11], photo detec-

tors [12–14], light emitting diodes (LEDs) [15,16], and

photo-electrochemical systems [17–20]. In fact, the fascinating

properties of TMDs can be revealed as they are thinned into a

monolayer or to a few layers thickness. The conversion from an

indirect to a direct band-gap semiconductor is one of these

prominent differences between the bulk and monolayer

TMDs. This phenomenon is due to quantum confinement

effects and has been found in different types of TMDs such

as MoS

[21,22], WS

[23], WSe

[24,25], and MoTe

[26–28]. This conversion makes 2D TMDs an excellent choice

for LED applications in the visible (Vis) and near infrared

(NIR) spectral regimes. Several different strategies have also

been employed to intensify and tune the photoluminescence

(PL) from 2D TMDs. Chemical treatments [29 –31], alloying

[32–35], coupling with surface plasmon polaritons [36,37], in-

tegration with photonic crystal cavities [38,39], and strain

engineering [40] are some of these proposed methodologies to

improve the band-to-band PL characteristics of 2D TMDs.

Moreover, it is found that a strong sub-band-gap PL can be

originated from defect-mediated transitions [23,25,41–46] that

are of great interest in the design of NIR LEDs.

Vanadium diselenide (VSe

) is another member of this fam-

ily that belongs to group five TMDs. Bulk VSe

is a layered

compound whose crystal structure is based upon strongly

covalent (intralayer) Se–V–Se bonds within each layer and

weak van der Waals (interlayer) Se…Se interactions in be-

tween. Compared to other types of TMDs, the existence of

an overlap between the valance band (VB) and conduction

band (CB) of VSe

makes this material a pure metal, and

it possesses a high level of electrical conductivity that is a

244

Vol. 6, No. 4 / April 2018 / Photonics Research

Research Article

key factor in optoelectronics applications [47–49]. Besides the

extra-high electrical conductivity of a VSe

monolayer (that has

been experimentally proved recently [47]), some other unex-

pected properties of this material have also been investigated

in recent years. Its unique charge density wave (CDW) [50],

ferromagnetism behavior [51–53], and electrochemical activity

[toward a hydrogen evolution reaction (HER)] [54–58] are ex-

amples of these explorations. Owing to its metallic nature with

no optical band gap, it has been expected that it should not be

considered for optical applications. Very recently, density func-

tional theory (DFT) calculations revealed that both bulk

2H-VSe

and 1T-VSe

and monolayers of H-VSe

and

T-VSe

are thermodynamically stable [48]. These DFT calcu-

lations demonstrated that although T-VSe

is a pure metal,

H-VSe

is a semimetal with no band gap. However, considering

spin-up or spin-down bands separately, H-VSe

introduces a

small band gap around 0.8 eV between the valance band maxi-

mum (VBM) and conduction band minimum (CBM). As the

only report on optical behavior of VSe

,Heet al. attained

an excellent photocatalytic activity toward methyl orange

degradation from bulk VSe

[59].

Another factor that limits the functionality of VSe

is its

synthetic challenge. Other types of nanostructured TMDs,

such as luminescent MoS

quantum dots and nanosheets,

can be obtained with a variety of controllable synthesis meth-

ods, such as chemical vapor deposition (CVD) [21,22] and

sonication-assisted liquid exfoliation [60–64]. However, for

the case of VSe

, researchers could only realize it by chemical

vapor transport (CVT) [65–67], CVD [47,68], and Scotch-

tape-based mechanical exfoliation [54] of the bulk VSe

, until

Xu et al. [53] recently synthesized it in an aqueous solution.

Later, other researchers have also proposed different chemi-

cal-based synthesis methods to make VSe

nanosheets [55,59].

However, due to the complex chemical environment in solu-

tion, it is still challenging to prepare contamination-free

samples by a solution-based method. One of the facial and

large-scale-compatible methods that has been recently used

to synthesize quantum dots (QDs) from TMDs and other

2D materials such as graphene is pulsed laser ablation (PLA)

[69–72]. We have recently demonstrated that by tuning

the laser fluence, blue and green ultra-small luminescent

silicon QDs can be synthesized [73]. However, up to now,

there is no report on the synthesis of VSe

nanostructures

using PLA.

In this paper, we propose a top–down, large-scale-compatible,

surfactant-free, and widely adopted approach to synthesize VSe

nanostructures (directly from the rock) by utilizing a two-stage

process. In the first stage, the nanosecond PLA technique is

utilized to produce three-dimensional (3D) VSe

nanoparticles

(NPs) from its bulk. Following this step, the ultra-sonication-

assisted lithium intercalation process is conducted using lithium

carbonate in a colloidal solution of NPs to successfully produce

2D VSe

nanosheets (NSs). During this process, ultra-sonication

assists lithium ions to easily permeate into the VSe

matrix and

break the weak van der Waals interaction between the stacked

layers. Afterward, the structural and optical characterizations are

conducted on the NP and NS samples. Our findings show that

while the VSe

rock is a metallic material with no band gap, the

resultant NPs and NSs are photo-responsive. It has been shown

that NPs have an effective band gap of 1.75 eV, and this value

gets wider to an amount of 3.45 eV for the case of NSs.

However, the main difference between NP and NS morpholo-

gies is raised from their PL properties. While NPs possess a weak

emission at the Vis-NIR regimes, NSs have a strong emission

with a peak located at around 710 nm. From these findings,

it can be speculated that moving from 3D NPs to 2D NSs, a

transition from the indirect to direct band gap takes place.

Moreover, the experimental characterizations elucidate the fact

that this emission is mainly originated from defect-mediated

transitions. The origin of all the above-mentioned phenomena

has been scrutinized and discussed in this study. To the best

of our knowledge, this is the first report on the synthesis of

luminescent VSe

NSs where its facile and contaminant-free

preparation route makes the upscaling possible. Besides its

inherently high electrical properties, this report proves that

VSe

can also provide a superior optical response, which makes

it a potential material for future optoelectronic applications.

2. EXPERIMENTS

A. VSe

Nanoparticle Synthesis

For the synthesis of VSe

NPs, the PLA method was employed

similar to our previously reported study [73]. Briefly, a Nufern

NuQ fiber laser (NUQA-1064-NA-0030-F1) operating at am-

bient temperature with a beam wavelength of 1064 nm, pulse

duration of 100 ns, repetition rate/frequency of 30 kHz, and a

pulse energy of 1 mJ was utilized for the process. To synthesize

the VSe

colloidal nanoparticles, pure VSe

rock is used as a

bulk target that is immersed in deionized water. The laser beam

scans the VSe

target (in an active area of 1cm

) with a spot

size of approximately 3.8 mm in diameter using a 200 mm focal

length taking into account the refraction through the water.

The laser ablation scan pattern is chosen to be an inward spiral.

The reason is the fact that, in spiral scanning, the ablated

nanoparticles in each step are collected toward the center

and re-ablated in the next cycles. Considering the fact that

ablation was carried out for 300 loops (at a fluence amount

of 50 mJ∕cm

), it is expected that the produced NPs would

undergo several melting/re-solidifying cycles, and this in turn

leads to the formation of defective small NPs. The output of the

PLA is an orange-colored solution.

B. VSe

Nanosheet Synthesis

The intercalation reactions are performed by adding 10 mg

into 2 mL VSe

NPs solution, which is roughly a

molar Li excess of 2:1. Then, the lithium intercalation is

facilitated by applyi ng ultra-sonication for 1.5 h. The duration

of sonication is chosen based on the color of the NS solution.

We found that after this duration, the color of solution is

turned into a homogeneous transparent gray.

C. Material Characterization

To characterize the structural properties of the synthesized

VSe

NPs and NSs, a scanning electron microscope (SEM,

FEI—Quanta 200 FEG) operated at 10 kV and a transmission

electron microscope (TEM, Tecnai G2-F30, FEI) operated

at 200 kV are employed. For TEM and high-resolution TEM

(HRTEM) measurements, a few droplets of the solution

Research Article

Vol. 6, No. 4 / April 2018 / Photonics Research 245

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