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.

J Mater Sci: Mater Electron Improved growth control of atomically thin WSe2 flakes using pre-deposited W source Van Tu Nguyen1,2,* , Ngoc Minh Phan1, and Ji-Yong Park3,* Graduate University of Science and Technology, Vietnam Academy of Science and Technology, Hanoi 100000, Vietnam Institute of Materials Science, Vietnam Academy of Science and Technology, Hanoi 100000, Vietnam Department of Physics and Department of Energy Systems Research, Ajou University, Suwon 16499, Korea Received: August 2021 ABSTRACT Accepted: 14 September 2021 The improvement in the growth yield and control of atomically thin WSe2 flakes by chemical vapor deposition (CVD) using pre-deposited WO3 nanopowders 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 WSe2 flakes are grown on the growth substrate with high density and an average size of around 20 lm The devices based on the as-grown WSe2 flakes show p-type behaviors with a high on/off ratio of * 105 and carrier mobility of * 0.5 cm2 V- s-1 as well as a large positive photoresponse The density and size of WSe2 flakes can be controlled 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 Ó The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021 Introduction After the introduction of graphene, diverse two-dimensional (2D) semiconductors such as transitionmetal dichalcogenides (TMDCs) and layered elemental materials (black phosphorus, silicene, tellurene, 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 applications 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 to E-mail: tunv@ims.vast.ac.vn; jiyong@ajou.ac.kr https://doi.org/10.1007/s10854-021-07049-0 J Mater Sci: Mater Electron strategies such as surface functionalization [8, 9], plasma treatment [10], charge transfer by molecular adsorption [11–13], and metal work-function engineering 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 reproducibility while CVD is a more effective way for the preparation of wafer-scale, highly crystalline WSe2 on 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 WSe2 is more challenging than S-based 2D materials such as MoS2 and WS2 since the reduction of selenium is weaker than sulfur In addition, the sublimation temperature of WO3 precursor 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 WSe2 as listed in Table However, this would produce a large number 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 Table The CVD synthesis of WSe2 from WO3 powder as W precursor 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 WSe2 in some cases [28, 29] Moreover, the formation of dense WSe2 monolayers 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 disappear in WSe2 multilayers [30–32] In this report, we propose another effective solution for WO3-based CVD growth of atomically thin WSe2 flakes 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 Experimental 2.1 Preparation of the growth substrate SiO2/Si substrates are first cleaned by dipping in piranha solution at 70 °C for h, followed by washing in DI water several times for the removal of piranha residues and drying with N2 blow For WO3 nanopowder deposition on the cleaned substrate, the required amount of WO3 nanopowder (99.8%, WO3 mass (mg) Temperature (°C) Substrate Flake size 150 250 100 300 10 40 10 875 875 900 925 850 875–925 950 SiO2/Si SiO2/Si C-plane sapphire Sapphire Sapphire SiO2/Si SiO2/Si Flake Flake Flake Flake Film Flake Flake (* 10 lm) and film (* 95 lm) (10–50 lm) and film (* lm) (* 15 lm) References [20] [21] [37] [47] [48] [49] [41] J Mater Sci: Mater Electron 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 WSe2 as shown in Fig 2.2 The growth of WSe2 The growth of WSe2 thin flakes is carried out using atmospheric pressure CVD The diagram of the system 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 growth substrate with pre-deposited WO3 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 heater 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 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 evaporation techniques Firstly, the electrodes are patterned by photolithography Then, e-beam evaporator is used for the deposition of titanium (1.5 nm)/gold Fig A schematic process for the preparation of WSe2 (48.5 nm) Finally, the fabrication process is completed by a lift-off process 2.4 Characterization techniques The results of the CVD growth are quickly characterized by optical microscopy (OM) The height profiles 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 optoelectronic properties of the WSe2 FET device are characterized using the Keithley 4200-SCS parameter analyzer Results and discussion 3.1 Effect of different W precursor setups on the formation of atomically thin WSe2 flakes Figure shows 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 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 WO3 powder, 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 Experimental section) as shown in Fig 2b The growth mechanism of WSe2 in the latter case will be discussed in the next section J Mater Sci: Mater Electron Fig Optical microscope images of WSe2 flakes grown a without and b with WO3 precursors on the growth substrate An AFM image in Fig 3a and its height profile in Fig 3b show triangular WSe2 flakes with a height of *1.1 nm, which is close to that of monolayer WSe2 in the previous study [15] A histogram of measured values for the thickness of WSe2 flakes as shown in Fig S2 Raman and PL techniques are useful to estimate 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 represents 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 Fig a An AFM image, b a height profile, c Raman and d PL spectra of an as-grown WSe2 flake as shown in a layers results in a Raman peak around 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 WSe2 flake 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 J Mater Sci: Mater Electron previously published WSe2 [15, 19] value for monolayer 3.2 Growth mechanism of WSe2 from predeposited WO3 nanopowder The mechanisms of the CVD growth of WSe2 from WO3 and Se powders are previously proposed by many groups [21, 37, 41] In short, WO3 first undergoes a reduction reaction with H2 and 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 temperature, WO3 - x reacts to Se on the growth substrate, 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 addition, we also use WO3 nanopowder to increase WO3 - x concentration with the following advantages: (1) the large surface area, enhancing the reduction of WO3 to WO3 - x suboxides (2) WO3 can be uniformly and easily dispersed in a solvent and be coated on the growth substrate Moreover, we recognize that keeping the feeding of WO3 - x close to the growth substrate is important due to following reasons In the conventional CVD setup, WO3 powder 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 suboxide 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 - x pieces as shown in Fig 4a, resulting in the growth of sparse and small WSe2 flakes as in Fig 2a On the contrary, when the pre-deposited WO3 nanopowder is used, the suboxides originated from the pre-deposited WO3 nanopowder at high temperature would mostly diffuse around on the surface of the growth substrate As a result, there are more dwelling WO3 - x species 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 WO3 nanoparticles on the surface Inset in Fig 4b is AFM image of the sample grown at 850 °C for short time (3 min, WO3 concentration 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 WSe2 flakes whereas uniformly large triangular WSe2 flakes appear in many regions as depicted in Fig 2b In this way, the pre-deposition of WO3 nanopowders on the growth substrate seems to result in the effective increase in the density of WO3 - x species on the surface, which increases the probability of reaction with Se and subsequent lateral growth We also tried to grow WSe2 with various concentrations of WO3 while other parameters are fixed As shown in Fig S3 (Supplementary Information), for the concentration of mg/ml, WSe2 can 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 WO3 nanopowder works effectively as a W source for the CVD growth of WSe2 and makes it easier to control the densities and size of WSe2 flakes 3.3 Growth of graphene/WSe2 vertical 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 limitation 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 MoS2 on graphene by J Mater Sci: Mater Electron Fig Schematic illustrations of a 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 - x species Inset is an AFM image of the sample grown at 850 °C for CVD In these cases, they increased the adsorption of the precursors (MoO3 - x and 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 hydrogen (H2) in the CVD process [43] However, the scalable growth of highly crystalline WSe2 on graphene is more challenging since the chance of formation and absorption of WO3 - x suboxides is much lower than that of MoO3 - x on graphene as mentioned in the introduction As we confirmed the advantage of pre-deposited WO3 as 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 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 WSe2 is found at some locations, most of them are monolayer Figure 5b is Raman spectrum of an as-grown graphene/ WSe2 heterostructure, 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 properties of the CVD-grown WSe2 flakes The electronic transport properties are investigated using back-gated WSe2 FET 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 Figure 6b shows the transport property (Ids vs 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 usually attributed to the charge trapping in the device, which originates from the absorbate molecules or defects in the material or substrates Device À Á mobility can be calculated from l ẳ dI ds =dVbg ẵL=WCox V ds ފ, where L and W are the length and 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, Vds is 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 is about 0.5 cm2 V-1 s-1, comparable to previous reports [21, 26] J Mater Sci: Mater Electron Fig a An AFM image, and b a Raman spectrum of the asgrown graphene/WSe2 vertical heterostructure Fig a Schematic view and OM image of the device b The transfer characteristics of a FET device based on an as-grown WSe2 on SiO2/Si substrate in the linear (black) and log (red) scale (Vds = 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= V, Vbg = V) (Color figure online) 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, À Á I ph ẳ I ds Light I ds Darkị, is plotted in the inset in Fig 6c and shows a strong dependence on the backgate 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 concentration in the conduction channel The temporal photoresponse (Ids vs 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 ẳ A1 et=t1 ị ỵ A2 et=t2 ị ; 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 J Mater Sci: Mater Electron stage are tr1 = * 4.2 s, tr2 = * 47 s, while the time constants for the decay stage are td1 = * 3.0 s, td2 = * 22 s These results agree well with the existing study on the WSe2 device [45] The response times (rise and decay time) display both fast and slow time components During the rise, the fast component is attributed 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 (photogating 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 photodetection 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 photoresponse time, defects passivation needs to be studied on both WSe2 and SiO2 substrates Conclusions We developed a CVD method with the direct predeposition of WO3 nanopowders on a growth substrate to synthesize atomically thin WSe2 flakes This approach shows several advantages: (1) more W supply with less WO3 source; (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 Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry and Energy (No 20184030202220) VTN acknowledges financial support from the Vietnam National Foundation for Science and Technology Development (No 103.992020.36) and Graduate University of Science and Technology under Grant Number GUST.STS.ÐT2020-KHVL01 Author contributions VTN: Conceptualization, Methodology, Investigation, 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 References G Zhang, S Huang, A Chaves, C Song, V Ozcelik, T Low, H Yan, Nat Commun 8, 14071 (2017) https://doi.org/10 1038/ncomms14071 Y Wang, G Qiu, R Wang, S Huang, Q Wang, Y Liu, Y Du, W.A Goddard, M.J Kim, X Xu, Nat Electron 1(4), 228–236 (2018) https://doi.org/10.1038/s41928-018-0058-4 L Tao, E Cinquanta, D Chiappe, C Grazianetti, M Fanciulli, M Dubey, A Molle, D Akinwande, Nat Nanotechnol 10(3), 227–231 (2015) https://doi.org/10.1038/nnano.2014 325 B Radisavljevic, A Radenovic, 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Electron previously published WSe2 [15, 19] value for monolayer 3.2 Growth mechanism of WSe2 from predeposited WO3 nanopowder The mechanisms of the CVD growth of WSe2 from WO3 and Se powders are previously... of WSe2 flakes grown a without and b with WO3 precursors on the growth substrate An AFM image in Fig 3a and its height profile in Fig 3b show triangular WSe2 flakes with a height of *1.1 nm, which