Solution-Based Property Tuning of Black Phosphorus - ACS Applied

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Solution-Based Property Tuning of Black Phosphorus Shuangqing Fan,† Wanfu Shen,† Jun Liu,† Haicheng Hei,† Ruixue Hu,† Chunguang Hu,† Daihua Zhang,† Xiaodong Hu,† Dong Sun,‡,§ Jian-Hao Chen,‡,§ Wei Ji,∥ and Jing Liu*,†

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†

State Key Laboratory of Precision Measurement Technology and Instruments, School of Precision Instruments and Opto-electronics Engineering, Tianjin University, No. 92 Weijin Road, Tianjin 300072, China ‡ International Center for Quantum Materials, School of Physics, Peking University, No. 5 Yiheyuan Road, Beijing 100871, China § Collaborative Innovation Center of Quantum Matter, Beijing 100871, China ∥ Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Material & Micro-Nano Devices, Renmin University of China, Beijing 100872, China S Supporting Information *

ABSTRACT: The air instability of black phosphorus (BP) severely hinders the development of its electronic and optoelectronic applications. Although a lot of effort has been made to passivate it against degradation in ambient conditions, approaches to further manipulate the properties of passivated BP are still very limited. Herein, we report a simple and low-cost chemical method that can achieve BP passivation and property tailoring simultaneously. The method is conducted by immersing a BP sample in the solution containing both 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) and triphenylcarbenium tetrafluorobor in a mixture of water and acetone (v/v = 1:1). After the treatment, the BP sample is functionalized with TEMPO, which not only efficiently passivates BP but also p-dopes BP to a degenerated density level of 1013 cm−2. The performance of the BP field effect transistor is improved after functionalization with a high Ion/Ioff ratio of 106 and carrier mobility of 881.5 cm2/(V·s). The functionalization-induced doping also significantly reduces the contact resistance between BP and the Cr/Au electrode to 0.97 kΩ·μm. Additionally, we observe a great reduction of BP electrical and optical anisotropies after functionalization. This chemical functionalization method provides a viable route to simultaneously passivate and tune the properties of BP. KEYWORDS: black phosphorus, functionalization, p-type doping, contact resistance, anisotropy

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INTRODUCTION

with physical doping methods, chemical doping can be conducted more efficiently by a low-cost solution-based process and does not involve any physical bombardment, which is preferred in many circumstances such as mass production of thin-film devices. Besides doping, chemical functionalization is also used to passivate instable 2D materials against degradation in ambient conditions.22,25−27 One of such instable 2D materials is black phosphorus (BP) which attracts tremendous research interests because of its unprecedented electronic, optoelectronic, and anisotropic properties. To prevent BP from degradation in air, various methods have been developed, including application of a native oxide,28,29 physical encapsulation by a layer of an air-stable material,30−32 and chemical functionalization.22,27 The first two passivation methods are reported to achieve excellent passivation results but show difficulty in further tailoring the material properties. In contrast, chemical functionalization of BP may simultaneously

Carrier doping has been one of the most commonly used approaches to tailor the electronic and optoelectronic properties of semiconducting materials, which is crucially important to achieve desirable device performances.1−8 Since the rise of graphene and other two-dimensional (2D) materials,9−14 intense research effort has been made toward effectively doping the atomically thin materials. Currently, besides electrical field-induced doping, there are mainly two types of methods to dope 2D materials including physical and chemical doping. The physical doping methods usually use gas plasma with high energy to physically bombard 2D materials,15−18 during which the 2D material atoms may be oxidized, replaced, or broken away from the material crystal by the gas molecules/atoms to modulate carrier density. Consequently, this method will possibly result in material defects and instable doping after long-term exposure in ambient conditions, which significantly degrades the device performances. On the other hand, the chemical doping method utilizes dopant molecules to functionalize 2D materials, and the charge transfer between the dopant molecules and 2D material leads to the shift of the Fermi level.19−27 As compared © XXXX American Chemical Society

Received: August 29, 2018 Accepted: October 29, 2018

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DOI: 10.1021/acsami.8b14887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of BP functionalization by the modification solution composed of TEMPO and [Ph3C]BF4 in acetone/water (1:1 v/v). (b) XPS and (c) Raman spectra of BP before and after modification (in 10 μM TEMPO and 10 μM [Ph3C]BF4). (d,e) AFM images of a functionalized BP flake after exposure in air for 0 (d) and 30 (e) days, respectively. Scale bars are 500 nm.

and acetone (1:1, v/v). The modified BP sample was then washed thoroughly by acetone and isopropanol before characterization by XPS, Raman spectroscopy, and AFM, which suggest the functionalization of TEMPO on the BP surface. The schematic of TEMPO functionalization on BP is illustrated in Figure 1a, in which TEMPO bonds to BP through N−O−P bonding. In the XPS measurements, the chemical states of pristine BP and modified BP were both probed with O 1s, N 1s, C 1s, and P 2p core levels (Figure 1b). In the O 1s XPS spectra, a peak at 534.4 eV was detected from modified BP but not from bare BP, which can be assigned to N−O−P bonding, indicating covalent bonding between P and TEMPO. In both N 1s and C 1s spectra, the modified BP sample shows an increased peak intensity at 401.533,34 and 285.4 eV,35,36 both of which correspond to the C−N bond of TEMPO, demonstrating the existence of TEMPO on BP. The Raman spectrum of modified BP (pink line in Figure 1c) provides additional potential evidence for covalent functionalization, in which the A1g mode, representing the out-of-plane P atom vibration,37 splits in contrast to bare BP (purple line in Figure 1c), likely due to the presence of functional groups attached to the P atoms. Figure 1d,e shows the AFM images of a functionalized BP flake which had been exposed in ambient air for 0 day (Figure 1d) and 30 days (Figure 1e), respectively. No obvious surface degradation was observed from Figure 1e (surface roughness was 0.694) as compared to Figure 1d (surface roughness was 0.767), indicating the excellent passivation effect of the surface functionalization. We also measured the thickness of a BP flake before and after chemical modification by AFM, which increased from 2.5 to 3.8 nm. The increment of the thickness is highly likely due to the surface functionalization (as shown in Supporting Information.

achieve both passivation and doping effects. Previous literatures reporting the chemical functionalization of BP are mainly focused on the functionalization mechanism and/or the passivation effect; however, the electrical properties of BP tuned by the functionalization has not been investigated in detail. In this work, we demonstrated that the functionalization of BP by 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) can simultaneously passivate and tune the properties of BP. We used X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and atomic force microscopy (AFM) to demonstrate the covalent functionalization and passivation effect of TEMPO on BP, the results of which indicated that BP remained stable in air for at least 1 month after TEMPO functionalization. We further studied the modulation of BP properties by TEMPO functionalization. Ultraviolet photoelectron spectroscopy (UPS) was used to measure the change of the BP work function induced by chemical functionalization, while BP field-effect transistors (FETs) were fabricated with blank and patterned current channels to inspect the transfer curve evolution as the modification duration increased. We also measured the reduced contact resistance between BP and the metal electrode after chemical modification by depositing a series of Cr/Au electrodes with gradient spacing on a single functionalized BP flake. Finally, both electrical and optical anisotropic measurements were performed to investigate the anisotropic change of the functionalized BP flake.

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RESULTS AND DISCUSSION Characterization of Surface Functionalization. The chemically modified BP samples were prepared by immersing pristine BP flakes in the modification solution comprising of 10 mM TEMPO and 10 mM [Ph3C]BF4 in a mixture of water B


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Figure 2. (a) UPS spectra of BP before and after modification (in 10 μM TEMPO and 10 μM Ph3CBF4). (b) Schematic of a BP FET with blank doping. (c) Evolution of the BP FET transfer curve as the modification time increased from 0 to 5000 s (in 100 nM TEMPO and 100 nM Ph3CBF4). The source−drain voltage was set at 100 mV. The orange ball curve represents the transfer curve of the device (treated in the modification solution for 5000 s) after N2/H2 rapid annealing. Inset: the optical image of this BP FET. Scale bar: 10 μm. (d) Calculated volume doping concentration as a function of the modification time.

respectively, q is the unit electron charge, and μ is the carrier mobility. The carrier mobility is calculated by μ = GmL/ (VDSCoxW), where Cox = 1.21 × 10−8 F/cm2 is the gate oxide capacitance of 285 nm thick SiO2 from the parallel plate model and Gm = dIDS/dVGS is the transconductance. The estimated doping densities of BP at −60 VGS increased from 8.7 × 1012 to 3.6 × 1013 cm−2 when the modification time increased from 1 to 5000 s and the reached degenerated doping level ultimately. The volume doping concentrations corresponding to extracted n2D values are plotted in Figure 2d as a function of functionalization duration, which achieved a degenerated doping concentration of ∼1019 cm−3 after 10 s modification. The transfer curve of the modified BP FET can be recovered to the original state (before modification) by rapid thermal annealing in a N2/H2 mixture at 340 °C for 1 min (dotted curve in orange color in Figure 2c). The reversibility of the transfer characteristic may be ascribed to the breakage of the P−O bond and thus leads to the sublimation of TEMPO at around 340 °C. This unique feature provides the capability to controllably and reversibly tune BP electrical properties. Moreover, reduced hysteresis (Supporting Information. Hysteresis of a BP FET before and after functionalization is shown in Figure S2) and enhanced linearity of output curves (Supporting Information. The output curves of a BP FET before and after functionalization are shown in Figure S3) were also observed after the chemical modification, suggesting an improved channel surface quality and Ohm contact with electrodes. Next, we fabricated a back-gated BP FET with a patterned current channel. The BP flake had a thickness of ∼4 nm and size of ∼5 μm in length and ∼10 μm in width. A flake of hexagonal boron nitride (h-BN) was transferred on top of the BP FET as a dielectric layer, on top of which Cr/Au (10/40 nm) was deposited to prevent the covered area from functionalization. The underlapped distance between the BN/Cr/Au covered area and source (drain) electrode was 130 nm (100 nm). Then, the device was modified in TEMPO

The AFM images of a BP flake before and after functionalization are shown in Figure S1). Investigation of Electrical Properties. To investigate the effect of TEMPO functionalization on BP electronic properties, UPS and FET device measurements were carried out. Figure 2a shows the UPS spectra of a pristine BP flake before (black line) and after (red line) 10 min modification treatment. Since the work function of the measured material can be calculated by subtracting the secondary electron cutoff from the radiation energy (radiation energy of He I is 21.22 eV), the work function of the pristine BP sample was calculated to be Φ = 4.06 eV, similar to previous reports.38−40 After 10 min immersion in the modification solution, the work function of the BP sample increased by around ΔΦ ≈ 0.91 eV, corresponding to TEMPO-induced p-doping. Figure 2b is the schematic of a back-gated BP FET with a blank current channel, which means that the entire channel was exposed to the modification solution, resulting in blank doping of BP. Figure 2c shows the transfer characteristics of the device, as it was modified in the solution composed of 10 μM TEMPO and 10 μM Ph3CBF4 for a period of time ranging from 0 to 5000 s. The functionalization produced strong p-type doping that increased both on- and off-state currents with the on-current eventually saturated, leading to a degraded current modulation behavior within ±60 V gate bias. The observed electronic property progress is consistent with the electrons transferring from BP to triarylcarbenium tetrafluoroborates during TEMPO functionalization, which results in p-type doping. We further estimate the 2D sheet charge density, n2D, of the device after each modulation duration by the following equation10,41,42 n2D =

IDSL qWVDSμ

(1)

where IDS and VDS are the drain current and voltage, respectively, L and W are the length and width of the channel, C


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Figure 3. (a) Schematic of a BP FET with patterned doping. The exposed regions were functionalized by TEMPO. Scale bar: 10 μm. (b) Evolution of transfer characteristics (VDS = 0.1 V) as the modification time increased from 0 to 120 min (in 10 nM TEMPO and 10 nM Ph3CBF4). Inset: optical image of this BP FET. (c) Hole mobility and on/off current ratio of the BP FET measured under back-gate bias when the modification time (in 10 nM TEMPO and 10 nM Ph3CBF4) increased from 0 to 200 min. (d) Schematic illustration of the band structure of the BP FET with patterned doping.

Figure 4. (a) Optical microscopy image, (b) SEM image, and (c) schematic of a multichannel-length device fabricated on a single BP flake to extract the contact resistance. The flake thickness was 17 nm. Scale bar: 10 μm. (d) Transfer curves of the devices with different channel lengths before (dashed lines) and after (solid lines) functionalization. (e) Resistance vs channel length at VGS = −40 V for the same BP before and after functionalization. (f) Gate bias dependence of the measured contact resistance before (red curve) and after (blue curve) functionalization.

ascribed to the “peak” band structure formed in the highly pdoped region as depicted in the lower row of Figure 3d. The evolution of the device figure of merits including on/off current ratio and hole mobility as functionalization time is presented in Figure 3c, which shows a similar trend as the BP FET functionalized by aryl diazonium.27 In the initial and intermediate modification, the on/off current ratio and the hole mobility increase simultaneously. At high level of functionalization, the electrical properties of the BP FET degraded by first decreasing the hole mobility (and thus conduction), which results in a peak on/off ratio value of 106. We further characterized the reduction of the contact resistance between BP and the metal electrode after TEMPO functionalization. A BP flake of 20 μm × 140 μm and ∼17 nm thick was exfoliated from a bulk BP crystal and then transferred to a SiO2/Si substrate. The contacts with a width of 2.7 μm

(10 nM) and Ph3CBF4 (10 nM) solution for measurement (the usage of nM concentration is to slow down the p-doping progress). Figure 3a,b (inset) show the schematic and optical images of the BP FET with a patterned current channel after modification, respectively. The exposed regions of the current channel were heavily p-doped, whereas the patterned region remained almost intrinsic owing to the protection of the pattern stack. Figure 3b shows the evolution of the device transfer characteristics with increasing functionalization. The TEMPO modification not only produced strong p-type doping as observed in the blankly modified device but also enhanced the current modulation (increased the on-state current and decreased the off-state current). The increase of current at onstate when hole is the majority carrier is mainly due to the reduced BP-electrode Schottky barrier (as shown in Figure 3d upper row), whereas the decreased off-state current may be D


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Figure 5. (a) Optical image of a 6 nm thick BP flake deposited with 12 electrodes each of which were separated by 30°. Scale bar: 10 μm. (b) Dynamic current measured along six directions of the BP flake plotted in the polar coordinate with increased modification time (in 10 nM TEMPO and Ph3CBF4) from 0 to 60 min. (c) Enlarged direction-dependent dynamic current of the BP flake before functionalization.

Figure 6. (a) Optical image of a mechanically exfoliated BP flake transferred onto a Si/SiO2 substrate. The two points (Point 1 and 2) marked in white number with different thicknesses were selected for RDM measurements. Scale bar: 10 μm. (b,c) RDM measurement results plotted in the polar coordinate along 12 directions on Point 1 and 2 of the BP flake, respectively, with increased modification time (in 10 μM TEMPO and 10 μM Ph3CBF4) from 0 to 100 min.

because it had already been heavily doped and thus the carrier density did not change significantly with the electrical field. Inspection of Electrical and Optical Anisotropies. Next, we explored the effect of the modification on both electrical and optical anisotropies of BP. To study the electrical anisotropy of modified BP nanosheets, 12 electrodes (10 nm Cr/40 nm Au) were deposited on a ∼6 nm thick BP flake via e-beam lithography in a spherical manner, each of which was separated by 30° as shown in Figure 5a. Current measurements between each electrode pair separated by 180° were plotted in polar coordinates in Figure 5b, which showed strong angle dependency. Before modification, the maximum and minimum current were observed at ∼150°/330° (0.28 μA) and ∼60°/ 240° (0.17 μA), respectively (Figure 5c), leading to an Imax/ Imin ratio of 1.6, which agreed well with the in-plane electrical anisotropic ratio reported previously in ref 44. Then, the device was immersed in the modification solution (10 nM TEMPO and 10 nM Ph3CBF4). As the modification duration increased, the current along the six directions all increased and the increasement reached more than 1 order of magnitude after 60 min modification. However, the Imax/Imin ratio of this device after 60 min modification decreased to 82% of its original value. The decreased anisotropic level may be explained by the increased density of states after doping, in which the difference of effective mass along different lattice directions is overridden by the increased electron−electron scattering. The slight rotation of the anisotropic direction may be attributed to the unbalanced contact resistance change of the 12 electrodes during chemical modification. To demonstrate that the anisotropic modulation is real, we further applied reflectance difference microscopy (RDM) to measure the optical anisotropy of the BP film before and after

were defined by e-beam lithography. The channel lengths ranged from 500 to 4500 nm. 10/40 nm Cr/Au deposited via e-beam evaporation was used as contact metals on the same BP flake. The optical image, scanning electron microscopy (SEM) image, and schematic view of the BP FETs are illustrated in Figure 4a−c, respectively. Figure 4d shows the transfer curves of the BP FETs before and after TEMPO modification with channel lengths of 3.5, 3, 2.5, and 1 μm, respectively. Before modification, all of the devices showed moderate gate modulation and ambipolar behavior. After chemical modification, the on-state current of all of the devices at gate bias of −40 V increased around 10 times, whereas the off-state current at gate bias of 0 V increased around 1000 times. The contact resistances for both intrinsic and modified BP with the Cr contact were extracted by the transfer length method (TLM) as shown in Figure 4e. As measured by the TLM at high negative gate bias (−40 V),43 the contact resistance of the modified BP (functionalized for 10 s in the modification solution) decreased by a factor of ∼4 from 3.75 to 0.97 kΩ·μm. This value is also 1.8 times lower than the reported contact resistance for a high work function metal Pd (∼1.75 kΩ·μm),43 which usually has a lower contact resistance with BP than the low work function metal Cr. Figure 4f presents the contact resistances of a BP FET before and after modification as a function of gate bias. The contact resistance of the intrinsic transistor showed a strong gate-dependent behavior with a decrease at lower gate bias, which was attributed to the accumulated carrier density in BP under the metal contacts as the electrical field increased. On the other hand, the contact resistance of the modified BP transistor did not change much as the gate bias swept from low to high E



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modification. RDM is an innovative characterization technique developed in our lab to identify the orientations of an anisotropic crystal, which measures the normalized reflectance difference (N) of two polarized portion of light, s and p, as light is normally incident on the sample. Therefore, the absolute value of the measured RDM signal achieves maximum when the polarization direction of s light matches either of the two principle axes of BP (because the polarization directions of s and p light and the two principle axes of BP are both perpendicular to each other, when s light matches one of the principle axes of BP, the p light will match the other principle axis of BP). Moreover, the maximum absolute value is positively related to the anisotropic level of BP. The working principle of RDM has been described in detail in previous reports45 and Experimental Section. Figure 6a shows the optical image of a mechanically exfoliated BP flake transferred on a SiO2/Si substrate for RDM measurements. The sample was then modified in the solution of 10 μM TEMPO and 10 μM Ph3CBF4 in the mixture of water and acetone.Figure 6b,c shows the RDM measurement results taken at Point 1 (thickness was ∼3 nm) and 2 (thickness was ∼5 nm) on this BP flake, respectively, by rotating the relative orientation between the reflectance difference microscope (polarization direction of s and p light) and BP flake for 12 angles as marked in red numbers in Figure 6a. Before modification, both points exhibited a maximum absolute value of Nmax at ∼0°, 90°, 180°, and 270°, corresponding to two BP principle axes (the formula to calculate N is described in Experimental Section eq 2). As the functionalization time increased, the absolute value of Nmax for both measured points decreased, indicating a decreased anisotropy level, which is consistent with the decrease of electrical anisotropy after functionalization. After 100 min modification, the absolute value of Nmax for Point 1 and 2 decreased to 30.5 and 45.7% of their original values, respectively. In addition, the angles corresponding to Nmax for Point 1 and 2 showed no obvious trend of rotation toward a certain direction as the modification time increased. Instead, the angles fluctuated around the original value which was likely due to the optical measurement deviation. In summary, we conclude that the chemical functionalization of TEMPO can efficiently tune the ratio of both the electrical and optical anisotropies of BP flakes.

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EXPERIMENTAL SECTION

Functionalization. Few-layer BP flakes were mechanically exfoliated from a bulk BP crystal and transferred onto a SiO2/Si substrate (thickness of SiO2 layer was 285 nm). The prepared BP samples were then immersed in the solution of TEMPO and Ph3CBF4 in a mixture of water and acetone (v/v = 1:1) with various concentrations for different time durations. After immersion, the samples were thoroughly washed with acetone and isopropanol and dried in N2 flow. Sample Characterization. XPS measurements were performed in ultrahigh vacuum in ESCALAB 250 Xi (Thermo Scientific, USA) with a nominal spot size of 400 μm. The UPS measurements were carried out in an ultrahigh vacuum chamber using a He 1α (21.2 eV) source. A negative bias of −5 V was applied on the sample. The XPS/UPS sample was prepared by ultrasonicating bulk BP in the mixture of water and acetone (v/v = 1:1) followed by adding 10 mM TEMPO and 10 mM Ph3CBF4 in the mixture. After setting the solution overnight, the precipitate was washed thoroughly by acetone and isopropanol and dried in N2 flow for XPS/UPS measurement. The spatial Raman peak intensity mapping was conducted on a Raman microscope (Renishaw inVia) equipped with a 532 nm laser (∼1.38 mW laser power, 0.5 μm lateral resolution, 10 s exposure time) and a 100× objective lens. The AFM measurement was conducted on a Dimension Icon atomic force microscope (Bruker, Germany) in tapping mode. BP FET Fabrication and Characterization. BP FETs were fabricated based on BP flakes transferred onto SiO2/Si substrates. Standard electron-beam lithography and electron-beam evaporation were used to deposit the Cr/Au (10/40 nm) contacts with a predefined geometry, followed by a lift-off process using acetone. The FET measurements were conducted using an Agilent parameter analyzer B1500A. All measurements were carried out under ambient conditions. Optical Anisotropic Measurement. RDM measures the normalized reflectance difference of two polarized portion of light, s and p, when light is normally incident on the sample. It is described as

2(rs − rp) Δr = r rs + rp

(2)

where rs and rp are the reflectance of s- and p-polarized light, respectively, r is the average reflectance of s and p light, and Δr is the reflectance difference between s and p light. When the relative orientation (θ) between RDM and an orthorhombic crystal varies, the measurement results N(θ) periodically changes with θ in the following way

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N (θ) =

CONCLUSIONS In conclusion, we demonstrated a simple and low-cost chemical method to effectively tune BP properties by TEMPO functionalization through a solution-based process. The functionalized BP sample was p-doped up to a doping density as high as ∼1013 cm−2. The degenerated doping led to an improved FET device performance and significantly reduced the contact resistance. Specifically, the BP FET after modification presented a high Ion/Ioff ratio of 106 and carrier mobility of 881.5 cm2/(V·s). The Cr-BP contact achieved an ultralow resistance value of 0.97 kΩ·μm. Moreover, the chemical functionalization was also found to tune the ratio of both electrical and optical anisotropies of the functionalized BP samples, which may be explained by the isotropic nature of the TEMPO assembly layer on a BP flake. The chemical functionalization-enabled manipulation of BP properties may open up new routes to efficiently tune the semiconducting properties of air-instable 2D materials.

2(rs − rp) rs + rp

=

2(rx − ry) rx + ry

cos(2θ) (3)

where rx and ry are the reflectance along the two principal lattice directions of the orthorhombic crystal. According to eq 3, for a perfect isotropic crystal, N(θ) is zero for all angles, whereas for anisotropic samples (BP), it reaches a maximum/minimum value when the RDM orientation matches the armchair/zigzag direction. Therefore, RDM can be used to evaluate the anisotropic level of BP and identify the directions of principle axes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b14887. AFM images; hysteresis of a BP FET before and after functionalization; and output curves of a BP FET (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. F


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Wanfu Shen: 0000-0002-3932-5457 Chunguang Hu: 0000-0001-6485-904X Daihua Zhang: 0000-0002-0163-0616 Dong Sun: 0000-0002-0898-4548 Wei Ji: 0000-0001-5249-6624 Jing Liu: 0000-0002-8993-4074 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Nature Science Foundation of China (no. 21405109) and Seed Foundation of State Key Laboratory of Precision Measurement Technology and Instruments (Pilt no. 1710).

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Solution-Based Property Tuning of Black Phosphorus - ACS Applied

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