Layer-by-Layer Sorting of Rhenium Disulfide via High-Density

Oct 4, 2016 - Isopycnic density gradient ultracentrifugation (iDGU) has been widely applied to sort nanomaterials by their physical and electronic str...
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Layer-by-Layer Sorting of Rhenium Disulfide via High-Density Isopycnic Density Gradient Ultracentrifugation Joohoon Kang,† Vinod K. Sangwan,† Joshua D. Wood,† Xiaolong Liu,‡ Itamar Balla,† David Lam,† and Mark C. Hersam*,†,‡,§,∥,⊥ †

Department of Materials Science and Engineering, ‡Graduate Program in Applied Physics, §Department of Chemistry, ∥Department of Medicine and ⊥Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: Isopycnic density gradient ultracentrifugation (iDGU) has been widely applied to sort nanomaterials by their physical and electronic structure. However, the commonly used density-gradient medium iodixanol has a finite maximum buoyant density that prevents the use of iDGU for highdensity nanomaterials. Here, we overcome this limit by adding cesium chloride (CsCl) to iodixanol, thus increasing its maximum buoyant density to the point where the high-density two-dimensional nanomaterial rhenium disulfide (ReS2) can be sorted in a layer-by-layer manner with iDGU. The resulting aqueous ReS2 dispersions show photoluminescence at ∼1.5 eV, which is consistent with its direct bandgap semiconductor electronic structure. Furthermore, photocurrent measurements on thin films formed from solution-processed ReS2 show a spectral response that is consistent with optical absorbance and photoluminescence data. In addition to providing a pathway for effective solution processing of ReS2, this work establishes a general methodology for sorting high-density nanomaterials via iDGU. KEYWORDS: Two-dimensional nanomaterials, liquid-phase exfoliation, iodixanol, cesium chloride, photoluminescence, photocurrent

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modifications.11,18−33 LPE can yield large quantities of 2D nanomaterials but ultimately lacks control over structural parameters such as thickness. The structural polydispersity of LPE dispersions is known to introduce numerous problems, particularly in electronic and optoelectronic applications,1−3,12,27,34,35 thus motivating efforts to develop postexfoliation, solution-based separation methods. Toward this end, isopycnic density gradient ultracentrifugation (iDGU), which was originally applied in biomaterial separations,36−38 has been adapted to nanomaterial dispersions including carbon nanotubes39−44 and low-density 2D nanomaterials11,20,21 to improve structural and electronic monodispersity. Thus, far, this technique has been limited to nanomaterials with buoyant densities in aqueous surfactant solutions lower than the standard density-gradient medium iodixanol (ρmax = 1.32 g/ cm3). With a buoyant density as high as ρmax = 1.9 g/cm3,37,38 cesium chloride (CsCl) would appear to be a natural candidate to extend the density range of iDGU. However, with a significantly lower viscosity than iodixanol, density gradients based on CsCl are relatively unstable,38 which hinders its utility for nanomaterial separations.

wo-dimensional (2D) nanomaterials present unique structures and properties for ultrathin electronic and optoelectronic device applications.1−6 Recently, rhenium disulfide (ReS2), one of the semiconducting transition metal dichalcogenides (TMDCs), has been shown to have an unusual distorted 1T structure with random stacking and weak interlayer coupling.7−10 Unlike other TMDCs, such as molybdenum or tungsten disulfide, whose bandgaps transition from indirect to direct in the monolayer limit,11,12 ReS2 possesses a direct bandgap in the bulk and as a monolayer, which provides advantages in high-gain photodetector applications.13−15 Although its band structure remains qualitatively unchanged as a function of thickness, ReS2 does show a subtle layer-dependent photoluminescence (PL) blue-shift due to quantum confinement effects.7,16 Previous work has shown that ReS2 nanosheets can be isolated by micromechanical7−10,13−15 and chemical exfoliation methods.17 Although micromechanical exfoliation provides high-quality nanosheets, its lack of scalability presents serious limitations for real-world applications. Conversely, while chemical exfoliation can produce larger quantities of nanosheets, this process drives a phase transition that necessitates subsequent thermal treatments to attempt to recover the original ReS2 phase.17 Liquid-phase exfoliation (LPE) is an alternative, scalable route for isolating 2D nanomaterials without chemical © XXXX American Chemical Society

Received: August 25, 2016 Revised: September 29, 2016

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Figure 1. (a) Experimental procedure to enrich few-layer ReS2 nanosheets. ReS2 powder is exfoliated in water with 2% w/v sodium cholate using tip ultrasonication (step 1) and then centrifuged at 7500 rpm for 10 min to remove unexfoliated ReS2 powder (step 2). The supernatant is subsequently collected and ultracentrifuged at 20 000 rpm for 30 min to sediment large nanosheets (step 3). The sediment is finally redispersed in water with 2% w/v SC. (b) Optical absorbance spectrum of the resulting ReS2 aqueous dispersion with a peak at 811 nm. Inset: photograph of the as-prepared ReS2 dispersion with the schematic illustrating ReS2 nanosheets surrounded by SC. (c) Atomic force microscopy image of solution-processed ReS2 following deposition on a Si wafer. (d) A schematic of the atomic structure of ReS2 (blue: Re atom; yellow: S atom; red dotted line: Re chain direction). (e) A transmission electron microscopy image of a ReS2 nanosheet and a high-resolution TEM image. The red arrow indicates the direction of a Re chain. (f) Selected area electron diffraction pattern of a ReS2 nanosheet. (g,h) X-ray photoelectron spectroscopy data for the (g) Re 4f and (h) S 2p core levels. (i) Raman spectrum of ReS2 nanosheets.

20 000 rpm to collect nanosheets with relatively large lateral sizes. The resulting optical absorbance spectrum is provided in Figure 1b, which shows a peak at 811 nm that is consistent with the PL emission position from previous reports.7 As-prepared ReS2 dispersions are dark brown in color (Figure 1b, inset). In Figure 1c, an atomic force microscopy (AFM) image of the solution-processed ReS2 nanosheets on Si confirms the ReS2 nanosheet morphology and the mechanical integrity of the dispersed flakes. For reference, a schematic of the ReS2 atomic structure is provided in Figure 1d, with the Re atoms (blue), S atoms (yellow), and Re chain direction (red arrow) indicated. The ReS2 structure of Figure 1d is verified with atomic-scale characterization by transmission electron microscopy (TEM). Specifically, a droplet of ReS2 solution is deposited onto holey carbon TEM grids for bright-field and high-resolution TEM (HRTEM) analysis. Figure 1e shows a low-resolution TEM image of a ReS2 nanosheet accompanied by a magnified HRTEM image. These images show a cleavage direction along the Re chains, which is consistent with literature precedent.10 Furthermore, in Figure 1f, the expected ReS2 crystal structure is

Here, we report the exfoliation and layer-by-layer iDGU sorting of high-density ReS2 in aqueous surfactant solutions. This work is enabled by density-gradient media based on mixtures of iodixanol and CsCl that combine the high viscosity of iodixanol with the high buoyant density of CsCl. Successful layer-dependent separation of ReS2 by iDGU is confirmed with a variety of characterization methods including atomic force microscopy (AFM), Raman spectroscopy, and PL spectroscopy. Solution-processed ReS2 thin films are also shown to provide a photocurrent response that is consistent with optical absorbance and PL spectra. Overall, this study provides a scalable means for producing optoelectronically functional ReS2 nanosheets with well-defined thicknesses in a manner that can be generalized to other high-density nanomaterials. In Figure 1a, the preparation steps for forming ReS2 aqueous dispersions are presented. Initially, ReS2 powder is exfoliated via ultrasonication in deionized water with the amphiphilic small-molecule surfactant sodium cholate (SC). The resulting dispersion is then centrifuged at 7500 rpm to remove unexfoliated ReS2 powder and then further ultracentrifuged at B

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Figure 2. (a) Solid-state PL spectrum of assembled ReS2 thin films. A pair of Voigt fitted curves at 1.47 eV (red) and 1.60 eV (green) indicate contributions from thick and thin nanosheets. (b) Liquid-phase photoluminescence excitation (PLE) plot of a ReS2 dispersion showing an emission peak at 757 nm. The water O−H Raman mode is the linear feature in the upper left corner of the PLE plot. (c) Wavelength-dependent I−V characteristics of a ReS2 thin film device in the dark and under illumination (L = 5 μm, W = 4380 μm). The spot size of the Gaussian beam is ∼0.36 mm2 (see Figure S5). Inset: optical image of the interdigitated electrode array preceding ReS2 thin film transfer. (d) Normalized photocurrent (Ipc) vs wavelength (blue: Vd = −50 V, red: Vd = 50 V).

confirmed with selected area electron diffraction (SAED), thus showing that the solution-processed ReS2 nanosheets maintain their original crystalline nature without phase transformations. The chemical state of the ReS2 nanoflakes are probed with Xray photoelectron spectroscopy (XPS). Core-level XPS spectra for Re 4f (Figure 1g) and S 2p (Figure 1h) show Re 4f7/2 and S 2p3/2 subbands at 42.7 and 163.1 eV, respectively, which are close to the expected values for bulk ReS2.45 Depth-profiled XPS measurements, presented in Figure S3, corroborate the existence of unoxidized ReS2. A Raman spectrum taken on solution-processed ReS2 is presented in Figure 1i, where six Raman modes (labeled I to VI) are observed at ∼137, ∼145, ∼153, ∼163, ∼215, and ∼238 cm−1, respectively. These modes further confirm the integrity of the solution-processed ReS2 nanoflakes and correspond to Eg-like (I to IV) and Ag-like (V and VI) vibrations.7 The semiconducting direct bandgap nature of ReS2 is investigated in Figure 2. In particular, Figure 2a shows a PL spectrum (532 nm excitation) from a ReS2 thin film assembled from solution. The PL spectrum possesses an emission doublet at ∼1.47 eV (red Voigt fitted line) and ∼1.60 eV (green Voigt fitted line), presumably related to PL emission from thick and thin nanosheets.7 These results are consistent with previous reports that have shown that the PL emission from ReS2 blue shifts in the ultrathin limit due to quantum confinement effects.7,16 Solution-based photoluminescence excitation (PLE) mapping is shown in Figure 2b for excitation wavelengths ranging from 400 to 540 nm and emission wavelengths ranging from 600 to 1000 nm. The emission peak at ∼758 nm (∼1.64 eV) for the ReS2 solution likely differs from the peak position for the thin film sample (Figure 2a) due to differences in the surrounding dielectric environment.3,23,46

Charge transport measurements are performed on the ReS2 thin films to verify that electronic and photoconductive properties are preserved following solution processing. Bottom-contacted ReS2 devices are fabricated with interdigitated electrodes on thermally oxidized Si wafers (inset of Figure 2c and detailed in the Supporting Information) and then annealed in ambient conditions at ∼100 °C for ∼10 min to improve flake−flake contacts. Symmetric, linear current− voltage (I−V) output characteristics at low biases suggest Ohmic contact between the Au electrodes and the ReS2 thin film (Figure S4). Nonlinearity in I−V curves at high bias can be attributed to space-charge limited transport. Transfer characteristics reveal negligible gating (on-to-off ratio of ∼1.1) due to high screening and thus weak field penetration in this film (thickness: 206 ± 23 nm). Nevertheless, the measured sheet conductivity of 1.65 × 10−2 S/m is among the highest reported for solution-processed nanomaterial thin films and exceeds that of solution-processed MoS2 thin films by 8 orders of magnitude.47 Figure 2c compares the wavelength-dependent dark and illuminated output characteristics for a ReS2 thin film device from 550 to 1050 nm. Figure 2d shows the photocurrent (Ipc = Ilight − Idark) as a function of wavelength at Vd = −50 and 50 V. The Ipc curves are normalized to an incident power of ∼1 μW for all wavelengths (starting power fluence is 5 × 105 W/ m2) by accounting for the measured power spectrum of the Xe arc discharge lamp irradiation source (Figure S5). The resulting photocurrent shows a peak at ∼800 nm in agreement with the optical absorbance (Figure 1b) and PL measurements (Figure 2a,b). While the aforementioned results show that ReS2 can be effectively exfoliated using aqueous surfactant solutions, the resulting ReS2 nanoflakes are polydisperse with respect to thickness and layer number. Consequently, it is of high interest C

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Figure 3. (a) Buoyant density model for SC-encapsulated ReS2 in aqueous solution, where N is the number of ReS2 layers, tReS2 is the thickness of a ReS2 layer, tA is the anhydrous layer thickness, tH is the hydration shell thickness, and σ is the packing density. (b) Buoyant density as a function of ReS2 layer number. The black line represents the expected buoyant density when ReS2 is encapsulated with SC in water. The red and green lines indicate the density limit of iodixanol and CsCl, respectively. (c) Density profiles of different density gradient media after ultracentrifugation.

Figure 4. (a) ReS2 layer-sorting iDGU process flow. After process steps 1−3 (Figure 1a), the collected ReS2 dispersion is placed onto a step gradient (step 4). After the concentration process (step 5), the ReS2 supernatant is placed in a linear density gradient (step 6). After iDGU, the ReS2 dispersion is separated by thickness and layer number (step 7). (b−e) Centrifuge tube photographs and corresponding density profiles for steps 4 to 7.

to adapt iDGU to the high-density limit to achieve layer-bylayer sorting of ReS2. Toward this end, Figure 3a shows a

geometrical buoyant density model that allows the constraints on the density-gradient medium for ReS2 iDGU to be D

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Figure 5. (a) ReS2 band separation in a centrifuge tube after iDGU with the buoyant density (blue) and concentration (red) of each fraction. (b) AFM images of ReS2 nanosheets from F4 and F10 on a Si substrate. (c) Fit of the buoyant density model to the experimental data. (d,e) Thickness and area histograms extracted from AFM measurements for F4 (red), F10 (green), and sedimented ReS2 (blue) with Gaussian fitting curves. (f) Raman spectra measured on F4, F10, sedimented ReS2, and concentrated ReS2. (g) PL spectra obtained from F4 (red), F10 (green), and sedimented ReS2 (blue) with overlaid Voigt fitting curves. The peak positions are at 1.58 (red), 1.53 (green), and 1.51 eV (blue).

determined.11,20,21 Specifically, the density profile for SCencapsulated ReS2 is given by ρ (N ) =

=

function of ReS2 layer number is shown in Figure 3b. Evidently, SC-encapsulated ReS2, even for monolayers, is predicted to possess a buoyant density than exceeds the iodixanol density limit (ρmax = 1.32 g/cm3) but is less than the CsCl density limit (ρmax = 1.9 g/cm3). While CsCl has sufficiently high density for ReS2 iDGU, the diffusion coefficient of CsCl (∼0.38 cm2/s) in aqueous solution is 10× higher than that of iodixanol (∼0.035 cm2/s) at room temperature,38,48 which implies that a linear gradient is formed throughout the entire solution after fewer than 3 h of ultracentrifugation (Figure S6). This observation is consistent with the Lamm equation,49 which describes the time evolution of the density profile (dχ/dt):

ρS N + 2mscσ + 2ρH O t H 2

(N + 1)t ReS2 + 2tA + 2t H

(1)

ρS N + 2mscσ + 2ρH O t H 2

(N + 1)t ReS2 + 2σVsc + 2t H

(2)

where ρS = 3.41 × 10−7 g/cm2 is the sheet density of ReS2, msc = 7.15 × 10−22 g is the mass of one SC molecule, σ is the surface packing density of SC on a ReS2 nanosheet, ρH2O = 1 g/ mL is the density of water, tH is the equivalent hydration shell thickness, and tA = σVSC is the equivalent anhydrous shell thickness of SC.11,20,21 The calculated buoyant density as a

⎤ dχ 1 d ⎡ dχ = − sω 2r 2χ ⎥ ⎢⎣rD ⎦ dt r dr dr E

(3) DOI: 10.1021/acs.nanolett.6b03584 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters where r is the distance from the center of rotation, t is time, s is the sedimentation coefficient, D is the diffusion coefficient, and ω is the rotor angular velocity. For the further illustration of the differences between CsCl and iodixanol, Figure 3c compares the experimental density profiles of CsCl (linear) and iodixanol (nonlinear). Because nonlinear density gradient profiles provide the best sorting efficacy, CsCl cannot be used directly as a replacement for iodixanol. In contrast, in the formation of a mixture between iodixanol and CsCl (as shown in Figure 3c) the desirable gradient nonlinearity of iodixanol and the higher maximum buoyant density of CsCl are achieved concurrently. Via the utilization of mixtures of iodixanol and CsCl, layersorting of ReS2 can be achieved by iDGU, as shown in Figure 4. In particular, ReS2−SC dispersions prepared according to Figure 1a (steps 1 to 3) are placed in a step density gradient (ρ = 1.54 g/cm3, step 4) and then ultracentrifuged to yield a concentrated ReS2−SC dispersion (step 5) with buoyant density less than 1.54 g/cm 3 . Following step 5, the concentrated dispersion is fractionated, and then CsCl powder is added to adjust the buoyant density to 1.58 g/cm3, after which the resulting solution is placed at the bottom of a linear density gradient (1.38 to 1.54 g/cm3, step 6). Subsequent ultracentrifugation drives ReS2 layer sorting (see the Supporting Information). It should be noted that ReS2 dispersions prepared with Pluronic F68 in place of SC precipitate after step 5 due to the low water solubility of the dispersant (Figure S7). Following iDGU, the upper part of the bottom gradient (ρ = 1.65 g/cm3) is mixed with the linear gradient with lower density, which results in an extended linear gradient (purple dashed line). However, the lower part of the high-density zone is also light brown in color because of thicker ReS2 flakes that possess a density greater than 1.55 g/cm3 that are not collected from step 3, result from diffusion from the nearest band during processing, or both. The ReS2−SC nanosheets form bands at their respective isopycnic points throughout the ultracentrifuge tube (step 7). A piston gradient fractionator is then used to recover the sorted ReS2 in 2 mm steps (F1 to F13). Figure 4b− e presents photographs of the centrifuge tubes and their respective density profiles for steps 4 to 7. Following iDGU, optical absorbance spectra for fractions F1 to F13 are measured (Figure S8) and used to extract the ReS2 concentration using Beer’s law. In Figure 5a, these concentration results (red line) along with the buoyant density (blue line) are plotted against fraction number. Fractions F4 and F10 have a higher concentration than adjacent fractions, consistent with the band positions in the photograph of Figure 5a. Figure 5b shows AFM images of F4 and F10 with average thickness values of 1.03 ± 0.11 nm and 2.09 ± 0.09 nm, respectively. The measured thicknesses are slightly larger than previously reported values for monolayer and bilayer ReS2,7 likely due to residual hydration layers and surfactants. On the basis of the measured thickness and buoyant density values, the ReS2 experimental data are plotted with the geometrical buoyant density model in Figure 5c. By fitting the density model to the experimental data, the values of σ and tH are determined to be σ = 1.4 × 1015 cm−2 and tH = 5.5 nm, respectively.11,20,21 The thickness and area histograms for F4 (red), F10 (green), and sedimented ReS2 (blue) with Gaussian fits are shown in Figure 5d,e, indicating that ReS2 separation is driven by thickness instead of lateral size as expected for iDGU. F4, F10, sedimented ReS2, and concentrated ReS2 are further examined by Raman and PL spectroscopy in Figure 5f,g. The Raman spectra indicate that the weak interlayer interaction for

ReS2 results in a decoupling of lattice vibration modes. In particular, no noticeable peak shift with layer number is observed, in contrast to thickness-dependent MoS2 Raman modes.11 However, the peak intensity of the IV mode (“inplane”, E2g-like) is decreased and the V mode (“out-of-plane”, A1g-like) is stiffened by ∼2 cm−1 with decreasing layer number, consistent with previous results.7 The PL spectra for F4, F10, and sedimented ReS2 occur at 1.58 (red), 1.53 (green), and 1.51 eV (blue), respectively, which is expected on the basis of previous reports that found ReS2 layer-dependent quantum confinement effects.7,16 Compared to other TMDCs such as MoS2 and WS2, the PL emission energy for ReS2 possesses a weaker thickness dependence, which further supports the weak interlayer interactions that have been postulated for ReS2.7,11 In conclusion, we have demonstrated stable ReS2−SC aqueous dispersions with concentrations in excess of 0.2 mg/ mL. The solution-processed ReS2 nanosheets are characterized with a comprehensive set of microscopic and spectroscopic methods, which confirm that the structure and properties of solution-exfoliated ReS2 are comparable to micromechanically exfoliated ReS2. Furthermore, solution-processed ReS2 thin films show significantly higher electrical conductivity than previously reported solution-processed TMDCs. Additionally, photocurrent measurements on ReS2 thin films show spectroscopic features that are consistent with optical absorbance and PL measurements. Despite the high buoyant density of ReS2, iDGU is used to achieve thickness sorting of ReS2 nanosheets by employing a mixture of iodixanol and CsCl as the density-gradient medium. Characterization of the resulting layer-by-layer sorted ReS2 dispersions show weak thickness dependence in the Raman and PL spectra, which confirms weak interlayer coupling in ReS2 compared to other TMDCs. Overall, this work demonstrates effective solutionbased exfoliation and thicknesses sorting for ReS2 that can be generalized to other high-density nanomaterial dispersions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b03584. Additional details on experimental methods. Figures showing powder XRD analysis, extinction coefficient measurements, XPS spectra with depth profiling, I−V curves of the photoconductive devices, optical irradiation source spectrum, density profiles of iodixanol and CsCl solutions with respect to centrifugation time, and optical absorbance spectra after iDGU. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

J.K. performed solution processing and AFM, Raman, and PL spectroscopy. I.B. also performed Raman and PL spectroscopy. V.K.S. collected photocurrent data. J.D.W. performed XPS and assisted in Raman and PL analysis. X.L. collected TEM, HRTEM, and SAED data. D.L collected XRD data. The manuscript was written through the contributions of all authors. M.C.H. supervised the project. Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS Solution processing was supported by the National Science Foundation (DMR-1505849), structural and chemical characterization was supported by the Office of Naval Research (N00014-14-1-0669), and charge transport measurements were supported by the National Science Foundation Materials Research Science and Engineering Center (DMR-1121262). This work made use of the NUANCE Center, which has received support from the NSF MRSEC (DMR-1121262), the State of Illinois, and Northwestern University. The XRD analysis made use of the J.B. Cohen X-ray Diffraction Facility, which is supported from the NSF MRSEC (DMR-1121262) and the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205). The Raman instrumentation was funded by the Argonne-Northwestern Solar Energy Research (ANSER) Energy Frontier Research Center (DOE DE-SC0001059).

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