Improved growth control of atomically thin WSe2 flakes using pre deposited w source

Improved growth control of atomically thin WSe2 flakes using pre deposited W source Improved growth control of atomically thin WSe2 flakes using pre deposited W source Van Tu Nguyen1,2, , Ngoc Minh Phan1, and Ji Yong Park3, 1Graduate University of Science and Technology, Vietnam Academy of Science and Technology, Hanoi 100000, Vietnam 2Institute of Materials Science, Vietnam Academy of Science and Technology, Hanoi 100000, Vietnam 3Department of Physics and Department of Energy Systems Researc.

Improved growth control of atomically thin WSe 2 flakes using pre-deposited W source

Van Tu Nguyen1,2,* , Ngoc Minh Phan1, and Ji-Yong Park3,*

1

Graduate University of Science and Technology, Vietnam Academy of Science and Technology, Hanoi 100000, Vietnam

2

Institute of Materials Science, Vietnam Academy of Science and Technology, Hanoi 100000, Vietnam

3Department of Physics and Department of Energy Systems Research, Ajou University, Suwon 16499, Korea

Received:1 August 2021

Accepted:14 September 2021

Ó The Author(s), under

exclusive licence to Springer

Science+Business Media, LLC,

part of Springer Nature 2021

ABSTRACT The improvement in the growth yield and control of atomically thin WSe2flakes

by chemical vapor deposition (CVD) using pre-deposited WO3nanopowders as

a W source is demonstrated WO3 nanopowders are pre-deposited on the growth substrate and utilized as a W source instead of separate W sources in the CVD system In this way, mostly mono or bilayer WSe2flakes are grown on the growth substrate with high density and an average size of around 20 lm The devices based on the as-grown WSe2flakes show p-type behaviors with a high on/off ratio of * 105and carrier mobility of * 0.5 cm2V-1s-1 as well as a large positive photoresponse The density and size of WSe2flakes can be con-trolled by adjusting the amount of pre-deposited WO3 nanopowders This approach can be used to grow W-based two-dimensional materials as well as their heterostructures with other materials such as graphene and carbon nanotubes

1 Introduction

After the introduction of graphene, diverse

two-di-mensional (2D) semiconductors such as

transition-metal dichalcogenides (TMDCs) and layered

ele-mental materials (black phosphorus, silicene,

tel-lurene, etc.) have been reported [1 3] Especially, 2D

TMDCs with large direct bandgaps are expected to

complement or replace graphene which has zero

bandgap in the electronic and optoelectronic

appli-cations Therefore, 2D TMDCs have been foci of

ongoing researches with various electronic and optoelectronic applications [4 6]

However, the majority of reported 2D TMDCs (MoS2, MoSe2, WS2, etc.) are intrinsically n-type due

to the strong electron doping from interfacial charge impurities and/or structural defects For example, MoS2, which is one of the most studied materials among 2D TMDCs and is regarded as a promising material with recent achievements in synthesis, characterizations, and applications, is also a naturally intrinsic n-type semiconductor [7] To build up basic circuit elements such as diode and transistor based

on MoS2, we also need p-type MoS2 Various

Address correspondence toE-mail: tunv@ims.vast.ac.vn; jiyong@ajou.ac.kr

https://doi.org/10.1007/s10854-021-07049-0

strategies such as surface functionalization [8, 9],

plasma treatment [10], charge transfer by molecular

adsorption [11–13], and metal work-function

engi-neering have been tried to convert MoS2 to p-type

with some success [14] However, these techniques

often require complicated processes An alternative

and simple way would be finding an intrinsically

p-type counterpart among other 2D materials

Monolayer WSe2, a member of the 2D-TMDC

family, is an example of such a p-type semiconductor

with a direct bandgap of 1.65 eV [15] Recently, many

attempts have been made to prepare monolayer

WSe2, mostly with mechanical exfoliation or chemical

vapor deposition (CVD) Mechanical exfoliation is

known as the simplest method to prepare good

quality monolayer WSe2 However, the samples are

usually of limited sizes and suffer from low

repro-ducibility while CVD is a more effective way for the

preparation of wafer-scale, highly crystalline WSe2on

various substrates For the CVD growth of WSe2,

diverse W sources such as W film, WO3 film, WSe2

powder, W(CO)6, WO3 powder have been utilized

[16–24] Among them, WO3 is the most common

precursor for the synthesis of large-area, high-quality

WSe2 with Se source However, the CVD growth of

WSe2is more challenging than S-based 2D materials

such as MoS2 and WS2 since the reduction of

sele-nium is weaker than sulfur In addition, the

subli-mation temperature of WO3precursor is higher than

that of MoO3[25] Therefore, it is more difficult to

control the W supply than the Mo supply during the

CVD process There have been attempts to overcome

this problem using a large amount of WO3 at high

temperatures (875–950 °C) to grow WSe2as listed in

Table1 However, this would produce a large

num-ber of by-products, which is not desirable Recently,

several groups have reported on the CVD growth of

WSe2 with the support of alkali halide salts (NaCl,

KCl, KI, KBr, etc.) [26–28] In this case, alkali halide

salts would react with WO3 and form the volatile tungsten oxychlorides (WOCl4, WO2Cl2, or both), tungsten oxyiodides, or tungsten oxybromides which has lower sublimation temperatures compared to that

of WO3 powder, thus facilitating the feeding of W sources However, in this case, the feeding of W source can become exceedingly significant for the CVD synthesis of WSe2, resulting in the formation of overlayer WSe2in some cases [28,29] Moreover, the formation of dense WSe2monolayers can bring great potential for both fundamental researches and applications in spintronics, electronics, photonics, and optoelectronics due to its direct band gap in the visible regime, valley degree of freedom, which dis-appear in WSe2multilayers [30–32]

In this report, we propose another effective solu-tion for WO3-based CVD growth of atomically thin WSe2flakes on SiO2/Si substrates While using WO3

nanopowder and Se powder as the precursors, WO3

nanopowders dispersed in IPA solvent are deposited

on the growth substrate prior to loading into the CVD system The resultant CVD growth results show dense WSe2 flakes with triangular shapes and an average size of 20 lm on SiO2/Si substrates, which are mostly monolayer with good crystalline quality This approach is also applied to the CVD growth of W-based 2D materials on graphene for the formation

of vertical heterostructures

2 Experimental

2.1 Preparation of the growth substrate

SiO2/Si substrates are first cleaned by dipping in piranha solution at 70 °C for 5 h, followed by wash-ing in DI water several times for the removal of pir-anha residues and drying with N2 blow For WO3

nanopowder deposition on the cleaned substrate, the required amount of WO3 nanopowder (99.8%, Table 1 The CVD synthesis

of WSe2from WO3powder as

W precursor

WO3mass (mg) Temperature (°C) Substrate Flake size References

size = * 100 nm, from Sigma-Aldrich) is weighed

and dispersed in IPA solvent by ultrasonication to get

a uniform solution Afterward, the solution is

deposited on the substrate by spin-coating Finally,

the sample is used for the CVD synthesis of WSe2as

shown in Fig.1

2.2 The growth of WSe2

The growth of WSe2 thin flakes is carried out using

atmospheric pressure CVD The diagram of the

sys-tem setup is presented in Fig S1a which is similar to

the previously published one for the growth of

MoS2[33–36] At the middle of the furnace, the

nanopowders and another SiO2 substrate with the

coated perylene-3,4,9,10-tetracarboxylic acid

tetrapotassium salt (PTAS) promoter are placed side

by side facing down on the top of a ceramic boat In

this way, there is a spacing of * 10 mm between the

substrate and the bottom of the quartz tube Another

ceramic boat with Se powder (300 mg) is located at

the upstream region of the heating zone, where the

sublimation temperature of Se is managed by a

hea-ter The synthetic process is schematically depicted in

Fig S1b Firstly, the CVD system is flushed with high

purity Ar (99.999%) flow rate of 300 sccm during

30 min for removal of any contaminant Then, the

furnace and the heater are heated up to 850 and

250 °C in a mixture gas of 100 sccm Ar/H2(96% Ar

and 4% H2) After growth time of 10 min, the furnace

is quenched down to room temperature in Ar gas

environment

2.3 Device fabrication

Field-effect transistors (FETs) devices are directly

fabricated on the as-grown WSe2 sample by

employing photolithography and e-beam

evapora-tion techniques Firstly, the electrodes are patterned

by photolithography Then, e-beam evaporator is

used for the deposition of titanium (1.5 nm)/gold

(48.5 nm) Finally, the fabrication process is com-pleted by a lift-off process

2.4 Characterization techniques

The results of the CVD growth are quickly charac-terized by optical microscopy (OM) The height pro-files of WSe2 flakes are obtained by atomic force microscopy (AFM) The structure, crystalline quality, and bandgap of WSe2 flakes are investigated by Raman and PL measurements with a 532 nm laser as

an excitation source The electronic and optoelec-tronic properties of the WSe2FET device are charac-terized using the Keithley 4200-SCS parameter analyzer

3 Results and discussion

3.1 Effect of different W precursor setups

on the formation of atomically thin

Figure2shows the differences in the growth of WSe2

flakes under the same CVD conditions [850 °C, 100 sccm mixture gas of Ar (96%) and H2 (4%), 10 min growth time] with different setups for W precursors When WO3 nanopowder as a W source is placed under the growth substrate in the ceramic boat for CVD growth, WSe2 flakes with small size and low density are typically grown as shown in Fig.2a Due

to the high melting point of WO3powder, not enough

W source is supplied to the growth substrate to get large WSe2 flakes On the other hand, denser WSe2

triangular flakes with the size of * 20 lm are observed when WO3 nanopowder is pre-deposited

on the growth substrate (as explained in the Experi-mental section) as shown in Fig.2b The growth mechanism of WSe2 in the latter case will be dis-cussed in the next section

Fig 1 A schematic process

for the preparation of WSe

An AFM image in Fig.3a and its height profile in

Fig.3b show triangular WSe2flakes with a height of

*1.1 nm, which is close to that of monolayer WSe2in

the previous study [15] A histogram of measured

values for the thickness of WSe2flakes as shown in

Fig S2 Raman and PL techniques are useful to

esti-mate layer numbers and crystalline quality of WSe2

Two representative Raman modes at * 250 cm-1

(E2g) and * 260 cm-1 (A1g) (Fig 3c) are observed

from an as-grown WSe2 flake The E2g peak

repre-sents the in-plane vibration while the A1g peak is

correlated with the vibration of selenium atoms in the

out-of-plane direction In the case of multilayered

WSe2, the Van der Waals force between adjacent

304–307 cm-1[15,37,38] The absence of this peak is usually used as a fingerprint of the monolayer WSe2

As shown in Fig 3c, there is no peak observed in the range, which indicates that the WSe2flake is indeed monolayer, which is also consistent with the AFM result The FWHM of the E2g peak, which is an indicator of the crystalline quality of the sample, is

*4.5 cm-1 This value is quite close to the reported ones for WSe2 crystal monolayers in the previous reports [39, 40] A PL spectrum from an as-grown WSe2 as shown in Fig.3d reveals a bandgap of

*1.61 eV (768 nm), which is consistent with the

Fig 2 Optical microscope

images of WSe2flakes grown

a without and b with WO3

precursors on the growth

substrate

Fig 3 a An AFM image, b a

height profile,c Raman and

d PL spectra of an as-grown

WSe2flake as shown ina

previously published value for monolayer

WSe2[15,19]

The mechanisms of the CVD growth of WSe2 from

WO3 and Se powders are previously proposed by

many groups [21,37,41] In short, WO3first

under-goes a reduction reaction with H2and H2Se, forming

an intermediate phase of WO3 - x and following by

the transportation onto the surface of the growth

substrate via the carrier gas At high growth

tem-perature, WO3 - x reacts to Se on the growth

sub-strate, leading to the formation of WSe2 nuclei and

the subsequent lateral growth Therefore, to obtain

large Wse2 flakes, a sufficient amount of WO3 - x

should be supplied continuously and adsorb on the

growth substrate during the growth To satisfy this

condition, we employ a PTAS seeding promoter,

which is known as a nucleation promoter in the CVD

synthesis of WSe2[20,42] It lowers the free energy of

nucleation, enhances the wettability of the growth

substrate, and further promotes the adsorption of

tungsten suboxide on the growth substrate In

addi-tion, we also use WO3 nanopowder to increase

WO3 - x concentration with the following

advan-tages: (1) the large surface area, enhancing the

reduction of WO3to WO3 - xsuboxides (2) WO3can

be uniformly and easily dispersed in a solvent and be

coated on the growth substrate Moreover, we

rec-ognize that keeping the feeding of WO3 - x close to

the growth substrate is important due to following

reasons In the conventional CVD setup, WO3

pow-der is usually placed on the bottom of a ceramic boat

while the growth substrate is facing down as

schematically depicted in Fig.4a At the growth

temperature (850 °C), the produced tungsten

subox-ide species would transport to the substrate surface

due to temperature gradient However, most of them

can be easily carried away from the surface of growth

substrate as the direction of the carrier gas flow is

perpendicular to that of WO3 - xpieces as shown in

Fig.4a, resulting in the growth of sparse and small

WSe2flakes as in Fig.2a On the contrary, when the

pre-deposited WO3 nanopowder is used, the

subox-ides originated from the pre-deposited WO3

nanopowder at high temperature would mostly

dif-fuse around on the surface of the growth substrate

As a result, there are more dwelling WO3 - xspecies

on the surface, thereby increasing the chance of reaction with Se to form WSe2 as schematically shown in Fig.4b Additionally, larger WSe2 flakes can be formed by Ostwald ripening process by small

WO3nanoparticles on the surface Inset in Fig.4b is AFM image of the sample grown at 850 °C for short time (3 min, WO3 concentration 5 mg/ml) In this case, the reduction and selenization process of WO3

are incomplete and we can observe white particles and overlayer at the center of triangular WSe2flakes whereas uniformly large triangular WSe2 flakes appear in many regions as depicted in Fig.2b

In this way, the pre-deposition of WO3 nanopow-ders on the growth substrate seems to result in the effective increase in the density of WO3 - xspecies on the surface, which increases the probability of reac-tion with Se and subsequent lateral growth We also tried to grow WSe2 with various concentrations of

WO3while other parameters are fixed As shown in Fig S3 (Supplementary Information), for the con-centration of 1 mg/ml, WSe2can be grown but they are quite small and low density As higher WO3 nanopowder concentration (5 mg/ml) is utilized, WSe2 flakes become larger with high density WSe2

films are observed when a higher concentration of

10 mg/ml is used This result also confirms that the pre-deposited WO3nanopowder works effectively as

a W source for the CVD growth of WSe2and makes it easier to control the densities and size of WSe2flakes

heterostructures

Vertical heterostructures of 2D materials such as monolayer TMDCs and graphene have been actively investigated and there have been many attempts to grow them by CVD [19, 36] However, previous reports have shown that there is a fundamental lim-itation on the CVD growth of TMDCs on graphene compared to the traditional substrates such as SiO2/

Si, quartz, and sapphire Due to the weak adsorption

of the precursors on the graphene surface, both the density of seeds and the lateral growth rate of TMDCs on graphene are very low Consequently, TMDCs with sub-mm sized or multilayer flakes tend

to form on graphene when grown by CVD Recently, some attempts have been conducted to obtain high coverage of atomically thin MoS on graphene by

CVD In these cases, they increased the adsorption of

the precursors (MoO3 - xand S) by distributing MoS2

seeds on graphene by combining PTAS promoter and

the treatment of graphene [36] or controlling the

nucleation of domains by the introduction of

hydro-gen (H2) in the CVD process [43] However, the

scalable growth of highly crystalline WSe2 on

gra-phene is more challenging since the chance of

for-mation and absorption of WO3 - xsuboxides is much

lower than that of MoO3 - x on graphene as

men-tioned in the introduction

As we confirmed the advantage of pre-deposited

WO3as a W source, we applied the same method to

synthesize WSe2 on graphene to form a vertical

heterostructure of two materials The resultant

growth is shown in Fig.5 The AFM topographic

image shows that the graphene surface is covered by

high-density WSe2 flakes with triangular shapes as

shown in Fig.5a Although bilayer WSe2is found at

some locations, most of them are monolayer

Fig-ure5b is Raman spectrum of an as-grown graphene/

WSe2heterostructure, including typical peaks of both

graphene and WSe2 Especially, a strong background

of underlying graphene is observed at high

wavenumbers, which stems from the PL of WSe2

More effective supply of W due to the pre-deposited

WO3 on graphene seems to enable the growth of

large-area WSe2 on graphene similarly on SiO2/Si

substrates

3.4 Electronic and optoelectronic

flakes

The electronic transport properties are investigated using back-gated WSe2FET devices, which are made

of the as-grown WSe2 sample Figure 6a is an OM image of the device with a triangular WSe2 flake between two electrodes Figure6b shows the trans-port property (Idsvs Vbg) of the device in the linear (black curve) and log scales (red curve) The device shows p-type behavior with an on/off ratio of * 105 Besides, a large hysteresis is often observed as shown

in Fig S4 when a gate voltage is swept This is usu-ally attributed to the charge trapping in the device, which originates from the absorbate molecules

or defects in the material or substrates Device mobility can be calculated from l ¼ dIds=dVbg

 L= WCð oxVdsÞ

width of the device channel, respectively, Cox= 11.5

nF cm-2(for 300 nm SiO2) is the capacitance between the channel and the back gate per unit area, Vdsis the bias voltage between source and drain, and dIds/

dVbg is the slope of the transfer curve in the linear region The calculated carrier mobility of the device shown in Fig.6is about 0.5 cm2V-1s-1, comparable

to previous reports [21,26]

Fig 4 Schematic illustrations

ofa a conventional and b a

current CVD setup for the

growth process of WSe2

flakes, which show differences

in the evaporation and

transport of WO3 -xspecies

Inset is an AFM image of the

sample grown at 850°C for

3 min

The optoelectronic property of the device is also

examined by measuring its transfer characteristics

under light illumination using a halogen lamp The

devices display a positive photoresponse in the

whole gate voltage range as shown in Fig.6c The

photocurrent, defined as the difference between

source–drain current with and without light,

Iph ¼ Ids Light

 IdsðDarkÞ, is plotted in the inset in Fig.6c and shows a strong dependence on the

back-gate voltage At the same time, the threshold voltage

(VT) largely shifted to a more positive voltage This

implies that photogenerated electrons are trapped,

acting like an effective negative gate voltage

(photogating effect) leading to increased hole con-centration in the conduction channel

The temporal photoresponse (Idsvs t) of the device

is also obtained as shown in Fig.6d for several cycles

of alternating dark and bright states, in which each cycle of ‘‘on’’ and ‘‘off’’ times lasted for 50 s In Fig.6d, both rising and decay of photocurrent are shown as a function of time and could be described

by bi-exponential functions: [44]

y ¼ A1eðt=t1 Þþ A2eðt=t2 Þ; where A1 and A2 are constants, t1 and t2 are time constants By fitting the photocurrent with the above function, we evaluated the time constants for the rise

Fig 5 a An AFM image, and

b a Raman spectrum of the

as-grown graphene/WSe2vertical

heterostructure

Fig 6 a Schematic view and

OM image of the device

b The transfer characteristics

of a FET device based on an

as-grown WSe2on SiO2/Si

substrate in the linear (black)

and log (red) scale (Vds=

2 V).c The transfer

characteristics of the device

with and without light

illumination The inset exhibits

the generated photocurrent of

the device as a function of the

gate voltage.d The temporal

photoresponse of the device

under cycles of ‘‘on-off’’ light

illuminations (Vds= 2 V,Vbg=

0 V) (Color figure online)

stage are tr1 = * 4.2 s, tr2 = * 47 s, while the time

constants for the decay stage are td 1= * 3.0 s, td 2= *

22 s These results agree well with the existing study

on the WSe2device [45] The response times (rise and

decay time) display both fast and slow time

compo-nents During the rise, the fast component is

attrib-uted to the photogeneration of electron-hole pairs in

the channel (photoconductive effect) under light

illumination while the slow one can be due to the

trapping of photogenerated electrons, which induces

more holes, thereby increasing conductivity

(photo-gating effect) During the decay process when light is

turned off, the electron–hole recombination results in

the fast decay, followed by a slow one due to the

discharging of the trapped electrons The slow

response time due to photogating-based

photode-tection is usually about seconds to tens of seconds

[45,46] This result is consistent with that obtained

from the photodetector devices based on other

TMDC materials For improvement in the

photore-sponse time, defects passivation needs to be studied

on both WSe2and SiO2substrates

4 Conclusions

We developed a CVD method with the direct

pre-deposition of WO3 nanopowders on a growth

sub-strate to synthesize atomically thin WSe2flakes This

approach shows several advantages: (1) more W

supply with less WO3source; (2) an easier control of

the WSe2 growth density and size; (3) synthesis of

W-based TMDCs/graphene heterostructures This

method can be applied to the growth of other

W-based 2D materials

Acknowledgements

This work was supported by ‘‘Human Resources

Program in Energy Technology’’ of the Korea

Insti-tute of Energy Technology Evaluation and Planning

(KETEP), granted financial resource from the

20184030202220) VTN acknowledges financial

sup-port from the Vietnam National Foundation for

Sci-ence and Technology Development (No

103.99-2020.36) and Graduate University of Science and

GUST.STS.ÐT2020-KHVL01

Author contributions VTN: Conceptualization, Methodology, Investiga-tion, Writing-review and editing NMP: Resources J-YP: Writing-review and editing, Supervision Declarations

Conflict of interest All authors declare that they have no conflict of interest

Supplementary Information: The online version contains supplementary material available at http s://doi.org/10.1007/s10854-021-07049-0

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