Recovery of the Pristine Surface of Black Phosphorus by Water

Article

Recovery of pristine surface of black phosphorus by water rinsing and its device application Suhyun Kim, Jong-Young Lee, Chul-Ho Lee, Gwan-Hyoung Lee, and Jihyun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 1, 2017

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ACS Applied Materials & Interfaces

Recovery of pristine surface of black phosphorus by water rinsing and its device application Suhyun Kim,1,‡ Jong-Young Lee,2,‡ Chul-Ho Lee,3 Gwan-Hyoung Lee*,2 and Jihyun Kim*,1 1

Department of Chemical and Biological Engineering, Korea University, Seoul 02841,

KOREA 2

Department of Materials Science and Engineering, Yonsei University, Seoul 03722,

KOREA 3

KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul

02841, KOREA E-mail: [email protected] and [email protected] ‡ These authors (S. Kim and J.-Y. Lee) equally contributed to this work. KEYWORDS: two-dimensional materials; black phosphorus; transistors; degradation; defects

ABSTRACT Black phosphorus (BP) has attracted a significant attention due to its excellent optical and electrical properties. However, the rapid degradation of BP under air ambient limits further 1

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research on its properties and implementation in various fields. This degrading behavior lowers the performance of BP-based devices and can even result in complete failure when exposed to air for extended period of time. In our research, the degraded surface with “bubbles” was recovered to its pristine state by rinsing with deionized water and following post-treatments. The formation of “bubbles” and their optical, morphological and electrical effects were systematically investigated by fabricating BP field-effect transistors (FETs) in conjunction with micro-Raman spectroscopy and atomic force microscopy. Water rinsing of the degraded BP flakes also allowed us to thin BP flakes down since phosphorus atoms are consumed while forming “bubbles”. Therefore, the recovery of the pristine surface not only results in a smooth and thinner morphology, but also improves the device performances. After the rinsing process, the field-effect mobility of the BP FET was maintained while a significant enhancement in the switching behaviors was achieved in conclusion. The capability of reversing the inevitable degradation that occurs once exposed to ambient condition can open up new opportunities for further applications of BP that was limited due to its instability.

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Introduction A facile way to obtain crystalline two-dimensional (2D) material by mechanical exfoliation method has drawn significant attention to 2D materials that are expected to play an important role in nanotechnology as they can be fabricated into atomic thickness with their unprecedented (opto)electronic and mechanical properties.1 Black phosphorus (BP) is one of these 2D materials that had been first synthesized in the 1910s, but it was only recently highlighted as a promising 2D material and its outstanding (opto)electrical properties have been revealed by ongoing researches;2-7 its carrier mobility and current on/off ratio can reach as high as ~1000 cm2/V·s and ~105, respectively.6,7 These high values can fill the gap between two of the most popular 2D materials, graphene and transition metal dichalcogenides (TMDCs). The carrier mobility of BP surpasses that of TMDC, while the ION/IOFF ratio exceeds that of graphene.3 This makes BP suitable for high frequency or thin-film electronics.7-9 Soon after its renaissance, the attractive characteristics of BP encouraged continuous characterizations and applications of BP-based (opto)electronic devices. The tunable direct bandgap of BP that varies from 0.3 eV for bulk to 2.0 eV for monolayer is attractive rather than other 2D materials including graphene and MoS2, which have no bandgap and large direct bandgap only for monolayer.3,10 Thus BP is an ideal candidate for near and mid-infrared optoelectronic applications.11,13 Despite all the advantageous properties, the lack of stability under ambient condition limits further study on BP and its implementation. The previous research on degradation of BP proved that BP starts to degrade immediately once exposed to oxygen, water and light, resulting in the formation of “bubbles” on the surface of BP.14-16 Especially oxygen plays a critical role in the degradation process and changing the properties of BP; theoretically, the 3



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oxygen molecules change thee BP surfacce hydrophillic, so that water can eeasily interract with the oxiddized surfacce.17 The deegradation pprocess has been obserrved as folloows. At firsst, small bubbless appear onn the surface of BP. It has been assumed a thaat the oxyggen moleculles react with phhosphorus atoms, a form ming phosphhorus oxidee. Due to th he hydrophiilic property y of BP, the watter moleculles can adsorb selectivvely on thee oxide surface and tuurn the pho osphorus oxide ccomponents into phosp phoric acid, 14,17,18 whicch is believeed to be thee bubbles observed o on the aair-exposedd BP. As thee exposure ttime increaases, the num mber of bubbbles increaases and they staart to coalessce. Eventuaally, they ennd up in a laarge bubble covering thhe whole su urface.1922

The iirreversible formation of bubbles deterioratess the perforrmance of B BP-based deevices.23

In ordeer to proteect or slow w this degraadation pro ocess down n under am mbient air, various encapsuulation metthods were proposed.222,24-28 The methods in nclude coveering BP with w airstable 22D materials such as grraphene andd hexagonall boron nitride (hBN) oor depositin ng Al2O3 layer byy atomic layyer depositiion.20,21,29-322 However, some of these encapsuulation meth hods are rather ccomplicatedd and additio onal defectss or contam mination can be inducedd during thee process. Theorettically, the degradation n is therefoore inevitab ble unless all a the fabriication proccess and operatioon of BP-based devicees are perfoormed undeer vacuum condition. Thus the means m to recoverr the pristinne BP surfface and ennhance the electrical properties of BP neeed to be proposeed.

Figure 1. Schemattic of experrimental proocedure inccluding UV treatment ((oxidation), rinsing with DII water and post treatm ment 4


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The schematic of the experimental procedure is shown in Figure 1. Here, we used a Hg lamp as a UV source in order to accelerate the degradation (oxidation) process in a controlled manner. After a certain height of the bubble covering the flake was intentionally created, the BP sample was rinsed with deionized (DI) water, followed by post-treatments, to remove the residual impurities. The thickness of BP was reduced simultaneously during the recovering procedures. There are several characteristics of BP that depend on the thickness; in addition to tunable direct bandgap that increases with decreasing the number of BP layers, electrical properties can also be enhanced by controlling the thickness. The change in the structural, optical and electrical properties of BP was monitored as we deliberately caused the degradation and went through the procedure to restore the properties of as-exfoliated BP flakes. Atomic force microscope (AFM), transmission electron microscopy (TEM) and Raman spectroscopy were used to analyze the structural and optical properties of BP. Then, field-effect transistors (FETs) were fabricated using BP flakes as the channel material. The electrical properties of FETs were compared after different treatments. A FET fabricated with a moderately thick BP flake (approximately ~40 nm) was subjected to a UV lamp (UVtreatment). The bubbles grew in size and thickness during exposure of BP to UV light. After performing UV treatment, the bubble eventually covered most of the flake. We deliberately waited until the surface of BP was entirely covered by the bubble because the partial removal of the bubble can result in uneven surface. Subsequently, the flake was immersed in DI water (DI rinsing) to rinse the bubble. The residual impurities, such as the adsorbed water and oxygen molecules, that still remain on the rinsed surface could be removed by vacuum annealing at 100 °C for 10 min. Finally, the pristine surface of thinned BP was recovered. We demonstrated how the recovery of BP surface from the degraded state can enhance the electrical performances of the BP-based electronics devices. 5



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Resultss and discussion

Figure 2. Optical microscopee images off BP flakes (a-e) before and after rinsing witth (f) DI water, ((g) IPA, (h) acetone, (i)) toluene annd (j) ethano ol (scale barr: 10 μm). s As previouusly reporteed, bubbles formed on BP surfacee are phosphhoric acid solution. Phosphorus atomss react with h oxygen m molecules to o form phosphorus oxxide speciess, which then tuurn into phosphoric accid in the presence of o moisture in the airr by the fo ollowing reactionns:18 5

∙ → 6

5 → 4

As phosphoric acidd is highly soluble andd easily ion nized in watter, the effecct on the bu ubble of DI wateer rinsing was w clearly observed.322 In additio on to DI waater, we havve also tested other solutionns to better define the bubble b resullting from degradation d and explainn the mechaanism of the bubbble removaal. We inten ntionally cauused enough h degradation on BP fl flakes for fo ormation of largee bubbles annd rinsed th he flakes in DI water, isopropyl i allcohol (IPA A), acetone, toluene, 6


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and ethanol. BP saamples weree immersedd in each sollution for 10 seconds. W We found that t only ving the bubbbles and th he rest of the solutions barely chan nged the DI wateer is effectivve in remov morphoology of thhe flake (Fiigure 2). T This supporrts the ideaa that the bbubble is made m of phosphooric acid, which w is alsso soluble iin alcohol but b not as much m as waater,32 thereefore the size of the bubblee was reducced only sliightly after rinsing witth alcohol ssolutions. Thus T we proceedded the folloowing experriments usinng DI waterr.

Figure 3. AFM im mages of (a) pristine (noon-contact (NC) ( mode), (b) and (cc) UV-treateed (noncontact (NC) modee and contacct (C) modee, respectiveely) and (d) DI-rinsed ((non-contacct mode) BP flakke. (e), (f), (g) and (h) Line L profile s that correspond to AF FM images (a), (b), (c)) and (d), respectiively. (i) Flaake heights of the flakees measured d at differen nt states.

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Figure 4. (a) Low w-magnificaation and (bb) high-maagnification TEM imagges and (c)) SAED pattern of pristine BP flake. (d) ( Low-maagnification and (e) hig gh-magnificcation TEM M images and (f) SAED patteern DI-rinseed BP flakess (scale: 10 nm, 1 nm, 2 1/nm from m left to rig ght) AFM meaasurements of an exfooliated BP flake f beforee and after UV treatm ment and rinsing with DI waater (Figuree 3) gave m more informaation on thee characteriistics of the bubble. wn in Figuure 3(a,e), 22 2 nm-thickk as-exfoliaated BP flaake showedd relatively smooth As show surface.. A few sm mall bubbles were obsserved on the t as-exfolliated BP su surface as they t can readily form withinn a short period of timee during AF FM measureement underr ambient condition. After U UV treatmennt for 6 miinutes, the BP surfacee was first scanned inn non-contacct mode (Figure 3(b,f)) in order to ob bserve the bbubble on BP B flake, which w is aroound 300 nm n high. Then thhe flake at the t same staate was scaanned in con ntact mode (Figure 3(cc,g)) to obsserve the morphoology of BP P surface beeneath the bbubble. AFM M tip in con ntact mode presses thee surface 8


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while scanning the sample, therefore the tip of AFM can penetrate the fluidic layer, i.e. the bubble in this case, to maintain the pressing force of 10 to 15 nN and measure the morphology of the underlying surface.33 This AFM result strongly implies that the bubble is a liquid phase. AFM images of the BP flake with bubble on the surface were also obtained in different modes: non-contact, contact and tapping modes in order (Figure S1). The first two scans in non-contact and contact modes measure ~540 nm high bubble and ~11 nm thick BP flake underneath, respectively. These results are consistent with AFM images in Figure 3. The repeated AFM scan in tapping mode showed that the bubble was still maintaining its shape after the prior measurement in contact mode. Therefore, we could confirm that the phase of the bubble is liquid. Finally, after the bubble was rinsed off with DI water, the measured thickness of the rinsed BP flake was around 11 nm, which is similar to the thickness measured by contact mode before rinsing process. Note that the DI-rinsed BP flake measured in non-contact mode has similar morphology (smooth surface) to that obtained of the UVtreated flake in contact mode (Figure 3(c,d)), suggesting that DI water effectively removes the bubble without damaging BP surface. When we tested DI-rinsing BP flake without UV exposure, the morphology of the flake showed no significant change (Figure S2), which confirms that DI water has no influence on the morphology of BP flake itself. TEM images and SAED patterns separately obtained from pristine (as-exfoliated) flake before any treatments and DI-rinsed BP flake after UV treatment (Figure 4) show that the high crystallinity of BP was maintained throughout our procedure. Thus UV irradiation causes layer-by-layer oxidation of BP and predominant phase transformation to phosphorus oxide at the top surface while maintaining the crystallinity of underlying BP layers.

In our

experiment, the BP flake was thinned down from 22 to 11 nm by UV exposure and the flat pristine surface of the thinned BP flake with high crystallinity was recovered by rinsing off 9



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the bubble with DII water.

s y (Ag1, B2g and Ag2) (aa) Normalized Raman sspectra, (b) average Figure 5. Raman spectroscop intensityy ratios of BP Raman modes to S Si peak and d (c) FWHM M obtained from a BP flake as exfoliatted, after UV V-treatmentt and DI-rinnsing. The opticcal propertties of BP P flakes were w analyzed by ussing micro o-Raman spectrosscopy. Thee evolution of Ramann spectra was w monito ored, then full width at half maximuum (FWHM M) and the intensity rratio of BP P-related Raaman modees to Si peak were obtained (Figure 5). 5 Since thee samples w were preparred by mech hanical exfo foliation of BP onto silicon dioxide/siliicon (SiO2/S Si) substrattes, the Ram man spectraa of Figure 5 were norrmalized with resspect to the intensity off Si peak (5521 cm-1) fo or each spectrum. The th three peaks at ~363, 440, annd 466 cm-11 correspond d to Ag1, B22g, and Ag2 Raman mo odes of BP,, respectively.13,34,36 The preevious repoorts on the optical o propperties of BP B have sho own that thee intensity ratios r of BP-relaated peaks are a critical parameterss in determ mining the properties p oof BP; the intensity i ratio deecreases as the numbeer of BP layyers is redu uced.34,37 To otal of 30 R Raman speectra (10 spectra each for prristine, UV--treated andd DI-rinsed BP flake) were w obtainned and the average for intennsity ratios and FWHM M were calcculated. Thee intensity ratio for eveery BP peak k (Figure 5(b)) deecreased in our experim ment, thereffore implyin ng that BP layers l have been etched d during UV irraadiation. Thhe intensity y ratios bareely changed d after rinsiing with DII water as stronger 10


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signal oof both BP and a Si substtrate could bbe detected after the bu ubble on topp was cleareed away. FWHM M (Figure 5(c)) also sh howed no cchange regaardless of UV U treatmeent and DI rinsing. These R Raman results indicatte the crysstallinity an nd reduced thickness of BP, wh hich are consisteent with AF FM and TEM M results (F Figures 3 and d 4).

Figure 6. IDS vs. VDS at diffeerent VGS oof (a) pristin ne, (b) UV--treated andd (c) DI-rin nsed BPbased F FET. IDS vs. VGS of (d)) pristine, ((e) UV-treated and (f) DI-rinsed B BP-based FET. F An identicaal FET device was used d for all the measuremeents. The back--gated FETs were thenn fabricated d using as--exfoliated BP as the channel materiaal to analyzee the electriccal propertiies of thinneed BP as illu ustrated in sschematic of o Figure 1. The eelectrical prroperties off the approxximately 40 nm-thick BP B device w were obtaineed under ambientt condition at room tem mperature aat each step p of 13 min n-UV treatm ment and DII rinsing (Figure 6). The opptical microscope imaages of the device at each measu sured state are also shown iin the loweer right inseets in Figuree 6(a-c). Drrain-source current (IDS ) was meaasured at drain-soource voltagge (VDS) fro om -50 to 500 mV by vaarying gate voltage v (VGS ) from -40 to 40 V 11



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with a step of 20 V. Transfer curves were obtained at a fixed IDS of 50 mV by sweeping VGS from -40 to 40 V. The linearity of IDS-VDS curves indicates the formation of the Ohmic-like contacts and the p-type characteristics could be observed from higher IDS measured at more negative VGS value.38 The as-exfoliated sample exhibited IDS of around -15 μA at VDS = -50 mV and VGS = -40 V, showing small gate modulation. The field-effect mobility and ION/IOFF ratio of the as-exfoliated BP-based FET were ~80 cm2/V·s and ~1, respectively. The field effect mobility was calculated using the following equation:39 =

∙ ∙

∙

where L, W, Cox, and gm are the channel length, channel width, capacitance, and transconductance, respectively. Although, the current level was maintained after UV treatment, the current was hardly modulated by the gate voltage, which is probably due to leakage current flowing through the bubble. After DI rinsing, the gate modulation was improved while the maximum current was reduced to around -8 μA because of removal of the bubble and reduction in the thickness of BP.

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Figure 7. IDS vs. VDS at differrent VGS of BP-based FET F (a) und der vacuum m condition prior to m annealing and (b) un nder ambiennt condition n after vacu uum annealling. IDS vs. VGS of vacuum BP-baseed FET (d)) under vaccuum condiition and (ee) under am mbient conddition after vacuum annealinng (c) fieldd-effect carrrier mobilityy and (f) ION o of the FET T at different states O /IOFF ratio analyzeed in Figurees 6 and 7. The ION/IOOFF ratios marked m with black circlles were caalculated when VGS = -40 ~ 40 V and the t blue squuare when VGS = -60 ~ 60 V. Vacuuum annealling was perform med at 100 °C for 10 minutes m andd the identiccal FET dev vice analyzzed in Figurre 6 was used. DI water did d remove the bubble,, however, the t residual impurities were still adsorbed a he device peerformance.. The identical BP deviice of Figurre 6 was to the surface and hindered th f 10 minu nutes to com mpletely subsequuently anneealed underr vacuum ccondition att 100 °C for removee the residuaal impuritiees (Figure 77). First, thee IDS-VDS an nd IDS-VGS (FFigure 7(a,b b)) were obtained under vaccuum condittion prior too annealing; the enhanccement of ggate modulaation and the decrrease of thee current weere clearly observed siince small bubbles b that at can act ass surface 13



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impurities were removed. After annealing, the device was removed from the vacuum chamber and analyzed under ambient condition. Figure 7(b,e) show significantly enhanced transistor characteristics. The field-effect mobility and ION/IOFF ratio were ~70 cm2/V·s and ~300, respectively. As shown in Figure 7(c,d), the carrier mobility was maintained while ION/IOFF ratio increased by two orders of magnitude in overall. The carrier mobility decreased to ~10 cm2/V·s after UV treatment, which can be attributed to carrier scattering by small bubbles.40 However, after DI rinsing and vacuum annealing, the bubbles and the residual impurities were completely removed, resulting in the recovery of the carrier mobility to its pristine level. Note that ION/IOFF ratio is highly dependent on the thickness of BP flake because the screening effects on the electric field become dominant for thicker BP flake, therefore decreasing the ratio.40 In addition, the bubbles on the surface can increase off current. As the vacuum annealing can remove the residual impurities physisorbed on the surface, higher ION/IOFF ratio could be achieved. Vacuum annealing was separately performed on another asfabricated (i.e. pristine BP) device (Figure S3), where the influence of vacuum annealing on both carrier mobility and current modulation was negligible. The degradation under air atmosphere was also observed after the treatments including UV irradiation, water rinsing and vacuum annealing. As-exfoliated BP flakes showed fast formation of bubbles, starting at ~ 12 h (a-c) (Figure S4). Meanwhile, the recovered flakes showed no bubbles until 60 h and the bubbles could be observed on some flakes after 84 h (d-f). This might be attributed to the formation of subnanometer stable oxide layers of P2O5 or additional functional groups of oxygen, which prevent charge transfer at the surface and slow down unstable oxide layer of POx.41 Therefore, chemical etching into pristine BP surface through UV treatment and DI rinsing attributed to the recovery, and even improvement, in the electrical properties of BPbased FET. This study can help solving the issues of inevitable degradation and the resulting 14


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bubble on BP surface during storage and fabrication process. Furthermore, rinsing the degraded part of the flake has the effect of reducing the thickness of BP through uniform and reproducible etching process.

Conclusion The degradation and recovery of pristine surface of BP by water rinsing and its device applications were investigated. The bubbles grew on BP flakes until they coalesced into a ~500 nm-thick bubble covering the surface, meanwhile, phosphorus atoms of BP were consumed for the formation of the bubbles and therefore, the thickness of BP was reduced as confirmed by the AFM images and the Raman spectra. The bubbles were then removed by immersing in DI water and drying. Although the surface morphology was relatively smooth (close to the pristine morphology) at this stage, the measured electrical properties revealed that the performances of BP FETs was still inferior to as-fabricated device due to the molecules introduced during the degradation and rinsing process. After annealing under vacuum condition, the electrical properties of the device were completely recovered due to the removal of the residual molecules; the carrier mobility of BP-based device was maintained at the same level and the ION/IOFF ratio was improved to ~300 by two orders of magnitude when compared to the pristine BP sample. Thus recovery process introduced in this study can successfully achieve excellent electronic properties that are comparable to pristine BP even after degradation had already taken place and these properties can be easily exploited in fabricating various BP devices without an air-tight system.

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Experimental Sample preparation Scotch-tape method was used to mechanically exfoliate BP flakes from high purity BP crystal pieces (Smart Elements). Then the flakes were transferred onto patterned Si substrates. The Si substrates consisting thermally grown 300 nm-thick SiO2 on top were pre-patterned by conventional photolithography for convenience of locating micrometer-sized flakes. The exfoliation process was performed inside a glovebox filled with argon gas to prevent pristine BP flakes from degrading under ambient condition.

Device fabrication In order to fabricate FETs using BP as the channel material, BP flakes were first mechanically exfoliated onto SiO2/Si substrates with back-gate made of Ti/Au (20 nm/80 nm) deposited at the bottom using an electron beam evaporator. Then BP flakes to be used for FETs were chosen and located using an optical microscope. The source and drain electrodes with the size of 100 μm × 100 μm were defined by a conventional photolithography process and Ti/Au (50 nm/100 nm) was deposited by electron beam evaporator to form the contacts.

UV irradiation and rinsing UV was irradiated onto the samples using a Hg lamp (Analamp, BHK Inc.) under ambient condition. The irradiating space was enclosed with a box to keep the oxygen radicals over a certain concentration. To recover the flat surface of BP flakes, the samples were immersed in 16


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DI water.

Raman spectroscopy The varying optical properties of BP flakes were observed through Raman spectroscopy (Renishaw, inVia) performed in backscattering configuration with a wavelength of 532 nm and a spot size of ~1 μm, where the laser power was 0.156 mW as high laser power may influence the degradation of BP, the peak shift or the FWHM of the phonon peaks. Raman spectra at the same point of the flake as exfoliated, after UV irradiation and rinsing with DI water were obtained.

Transmission electron microscope (TEM) BP flakes were transferred onto the copper grid with carbon mesh after liquid exfoliation in IPA. Then the structural characterization of the flakes was performed using TEM (JEOL, JEM-2100F) at an acceleration voltage of 200 kV.

Atomic force microscope (AFM) AFM (Park Systems, NX10) was used to observe the surface morphology and the thickness of BP flakes. Either non-contact, contact or tapping modes was used for each different case.

Electrical measurements 17



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A semiconductor parameter analyzer (Agilent, 4155C) and a probe station connected together were used to analyze the electrical properties of BP-based FETs. The current-voltage (I-V) characteristics were obtained as the device went through UV treatment, rinsing with DI water and annealing under vacuum (10-2 torr) at 100 °C for 10 minutes.

Supporting information. AFM images, I-V characteristics and optical microscope images of BP flakes

ACKNOWLEDGEMENTS The research at Korea university was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Korea (No. 20163010012140). G.H.L was supported by the Basic Science Research Program (2016M3A7B4910940) through the National Research Foundation (NRF) funded by the Korean government Ministry of Science, ICT and Future Planning. C.-H. L was supported by Basic Science Research Program (2014R1A1A2055112) through the NRF funded by the Korean Government Ministry of Education and KU-KIST Graduate School of Converging Science and Technology Program.

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Using

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P.; Coker, D. F.; Özyilmaz, B.; Castero Neto, A. H.; Fuhrer, M. S. Creating a Stable Oxide at the Surface of Black Phosphorus. ACS Appl. Mater. Interfaces, 2015, 7, 14557-14562.

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Table O Of Contents (TOC) grraphic

25


Recovery of the Pristine Surface of Black Phosphorus by Water

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