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Two-Dimensional Hydroxyl-Functionalized and Carbon-Deficient Scandium Carbide, ScCOH, a Direct Bandgap Semiconductor x
Jie Zhou, Xian-Hu Zha, Melike Yildizhan, Per Eklund, Jianming Xue, Meiyong Liao, Per O. Å. Persson, Shiyu Du, and Qing Huang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06279 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019
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Two-Dimensional Hydroxyl-Functionalized and Carbon-Deficient Scandium Carbide, ScCxOH, a Direct Bandgap Semiconductor Jie Zhou1+, Xian-Hu Zha1+, Melike Yildizhan2, Per Eklund2, Jianming Xue3, Meiyong Liao4, Per O. Å. Persson2, Shiyu Du1* and Qing Huang1* 1Engineering
Laboratory of Advanced Energy Materials, Ningbo Institute of Materials Engineering and Technology, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China. 2Thin Film Physics Division, Linköping University, IFM, 581 83 Linköping, Sweden. 3State Key Laboratory of Nuclear Physics and Technology, Center for Applied Physics and Technology, Peking University, Beijing 100871, China. 4Optical and Electronic Materials Unit, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan. + These authors contributed equally to this work. * Corresponding authors:
[email protected];
[email protected].
ABSTRACT Two-dimensional (2D) materials have attracted intensive attention in nanoscience and nanotechnology due to their outstanding properties. Among these materials, the emerging family of 2D transition metal carbides, carbonitrides, and nitrides (referred to as MXenes) stands out because of the vast available chemical space for tuning materials chemistry and surface termination, offering opportunities for property tailoring. Specifically, semiconducting properties are needed to enable utilization in optoelectronics, but direct bandgaps are experimentally challenging to achieve in these
2D
carbides.
Here,
we
demonstrate
the
fabrication
of
2D
hydroxyl-functionalized and carbon-deficient scandium carbide, namely ScCxOH, by selective etching of a layered parent ScAl3C3 compound. The 2D configuration is determined as a direct bandgap semiconductor, with an experimentally measured 1 ACS Paragon Plus Environment
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bandgap approximated to 2.5 eV. Furthermore, this ScCxOH based device exhibits excellent photoresponse in the ultraviolet-visible light region (responsivity of 0.125 A/W@360 nm/10 V, and quantum efficiency of 43%). Thus, this 2D ScCxOH direct-bandgap semiconductor may find applications in visible-light detectors, photocatalytic chemistry, and optoelectronic devices.
KEYWORDS: MXene, two-dimensional material, selective etching, DFT calculation, electronic properties, photodetector.
With the emerging technology of flexible and transparent electronics,1,2 two-dimensional (2D) semiconductors have attracted intensive attentions in recent years.3,4 Specially, direct bandgaps enable the applications of these 2D structures in optoelectronics. Typical 2D direct-bandgap semiconductors such as MoS2 and phosphorene were successfully fabricated at the beginning of this decade.4,5 However, the band gap of MoS2 is layer-dependent and the direct gap only appears in the monolayer limit;5 phosphorene is prone to chemical degradation upon exposure to ambient condition.6 Therefore, extensive efforts are devoted to optimize these 2D configurations and explore more 2D semiconducting members. 2D transition metal carbides have emerged as a recently established class of 2D materials7,8 with opportunities for tunability within a large chemical space of materials chemistry and surface termination, and corresponding property tailoring. Primarily, these materials are of the MXene family, so named because of their origin from the three-dimensional 2 ACS Paragon Plus Environment
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(3D) layered “MAX phases” and to emphasize the connection to graphene and other 2D materials. Here, M denotes early transition metal, A is mainly an element from groups 13-16, and X is C and/or N.9,10 The abundant chemical elements and versatile properties of MXenes have rendered them promising applications including energy storage,11,12
electromagnetic
interference
shielding,13
reinforcement
for
composites,14 water purification,15 and sensors,16 among others.8 Generally, these 2D carbides are metallic in nature, and many of them exhibit the unusual combination of conductivity and hydrophilicity. With appropriate termination Tx, a few MXenes may show semiconducting-like behaviors, notably Mo2CTx17 and Mo2TiC2Tx,18 with predicted indirect bandgaps.18,19 However, direct-bandgap semiconductors among the MXenes have been predicted theoretically,20-22 but remain a challenge to experimentally
realize.
Further
enriching
MXene
family
and
introducing
semiconducting members with direct bandgaps is critical for expanding the application of 2D carbides in optoelectronics. Previously, 2D scandium carbide is the only MXene member predicted to be a semiconductor regardless of functional group.21-23 Moreover, its hydroxyl-terminated configuration was predicted to be yield a direct-bandgap semiconductor.20,21 Theoretical work also found that both Sc2C(OH)2 and Sc2CF2 exhibit promising optical absorption in the visible-light region.24 The 2D configurations Sc2CCl2, Sc2C(SH)2, and Sc2NO2 were also predicted as semiconductors.25 However, for transition metal Sc, it is difficult to form the Al-containing MAX phases which are possible to be etched. Recently, by introducing Sc to the ternary Mo-Al-C system, 3 ACS Paragon Plus Environment
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out-of-plane ordered quaternary MAX phases (o-MAX) and in-plane chemical ordered MAX phases (i-MAX) were discovered,26 and the corresponding Mo2ScC2Tx27 and Mo1.33CTx28 MXenes were obtained. It is worth noting that selective etching of Al and Sc atoms occurs simultaneously in fluoride containing solution, which implies that it is difficult to produce the Sc-based MXene from the traditional approach. Moreover, although scandium containing layered carbides, such as Sc2Cl2C29 and Sc2B1.1C3,30 have been developed many years ago, there are no reports on exfoliation of these compounds. Thus, it is still a challenge to synthesize full Sc-based MXenes from selective etching of traditional MAX phases in the corrosive fluoride-containing solution. Structural design of a layered precursor can be etched and establishment of mild and compatible etching chemistry could be key issues for the synthesis of scandium-based MXene. As inspired by our previous works using non-MAX-phase layered carbides as precursors for MXene synthesis,31,32 here we demonstrate a 2D hydroxyl-terminated and carbon-deficient scandium carbide, namely ScCxOH, synthesized through selective etching of an alternative ScAl3C3 precursor, using an alternative organic base tetramethyl ammonium hydroxide (TMAOH) as etchant. In a combination of experimental studies and first-principles density functional theory (DFT)33 calculations, the underlying mechanisms of the selective etching process, and the structural, electronic and photoelectric properties of the obtained 2D carbide are investigated. This 2D material is determined as a semiconductor with a direct band gap of ~2.5 eV, which may have applications in ultraviolet-visible-light detectors, photocatalytic chemistry, and optoelectronic 4 ACS Paragon Plus Environment
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devices. Moreover, the structure of ScCxOH is beyond the traditional MXene configuration, which further expands the family of 2D carbides.
RESULTS AND DISCUSSION The layered ternary ScAl3C3 precursor was synthesized by an in situ reactive pulsed electric current sintering process. The selective etching process was implemented by using an aqueous organic base solution TMAOH. X-ray diffraction (XRD) results of the as-synthesized powders are shown in Figure 1 (black curve), where the phase composition as determined by the Rietveld refinement (Figure S1 and Table S1 in Supporting Information, SI) method was 88.96 wt.% ScAl3C3 with secondary phases, Sc2OC (5.26 wt.%), and residual graphite (5.78 wt.%). The reliability indices for the final result were Rwp = 12.1 %, RP = 8.7 %, and χ2=1.77, respectively. The in-plane and out-of-plane lattice parameters, a and c, determined from Rietveld refinement, are 3.350(1) Å and 16.776(1) Å, respectively, which are close to the values reported previously.34 The XRD pattern of the powders (in their vacuum-dried state), after etching in the TMAOH solution for about 72 h at 30-40℃, are shown in Figure 1 (red curve). The peak intensities originating from the parent ScAl3C3 decreased substantially, and a peak emerged at a lower angle of 2θ = 5.3° (Figure 1a-b). This can be attributed to an expanded lattice spacing of 16.70 Å, compared to the original 8.39 Å (2θ = 10.5°) of the ScAl3C3 crystal. This low-angle (0002) peak is typical for most reported HF-etched MXenes,35,36 and also typical for TMAOH-treated Ti3AlC2.37 Moreover, unlike the formed broad peaks typical for 5 ACS Paragon Plus Environment
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HF-etched MXenes, the observed lower-angle (000l) peak here is located at a relatively lower angle which corresponds to a larger lattice spacing, suggesting possibly spontaneous intercalation of TMA+ ions.38 Note that the ScAl3C3 powders were not fully etched after 72 h in the TMAOH solution, hence a small amount of residual graphite can also be detected in Figure 1. The peaks corresponding to residual Sc2OC vanished upon etching. The as-synthesized ScAl3C3 crystallites show typical plate-like morphology for layered materials with a lateral size of 5-10 μm (Figure S2). In contrast, apparent layer swelling and exfoliation of individual grains along the basal planes (Figure 1c) were observed after etching. The obtained Sc:Al ratio was about 1.00:0.89 (Figures S3a-b) from the as-etched powders with an area of about 2000 μm2, implying approximate 70% of the ScAl3C3 phase was converted into 2D ScCx. Notably, a certain amount of Al(OH)4- groups retained in the as-etched powders, thus the real converstion ratio could be higher. Sonication and centrifugation of the etched powders resulted in the separation of the flakes and formation of a stable colloidal solution in degassed deionized water (Figure S3c). A bright-field TEM micrograph of the delaminated flakes is shown in Figure 1d. Thin folded flakes can be observed, indicating that the flakes are mechanically flexible, in contrast to the relatively flat morphology with large in-plane stiffness typical of previously reported MXenes.35,36 The corresponding HRTEM results imply that the 2D ScCx flakes show good crystallinity and structural integrity in the basal plane (Figures S4e-f). The corresponding selected area electron diffraction (SAED) pattern (inset in Figure 1d) confirms the hexagonal symmetry of the as-exfoliated layers. The EDS analysis of the 6 ACS Paragon Plus Environment
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flakes (Figure S3c) shows the presence of Sc, C, O, N, and weak Al signal (which stems from the Al(OH)4- surface groups as discussed later). The molar ratio of Sc:C:O:N approximates 1.00:0.85:1.22:0.09, which indicates selective etching of the Al-C layer from ScAl3C3 and formation of a 2D ScC sheet with possible carbon vacancies during the etching process. The measured value of the in-plane lattice parameter, a, is 3.30 Å that is approximately that of the original ScAl3C3 crystal (3.35 Å), showing that the hexagonal crystal structure of the ScAl3C3 basal planes was inherited by the 2D sheets. Further, the AFM height profile (Figures S5a-b) implies that the delaminated flakes have an average thickness of about 2 nm, demonstrating that the obtained material is indeed two-dimensional.
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Figure 1. Synthesis and characterization of 2D ScCxOH. (a) X-ray diffraction patterns of ScAl3C3 powders before and after etching in an aqueous TMAOH solution. (b) detailed view with 2θ between 5° to 15°, where background was subtracted. (c) SEM image of ScAl3C3 crystallites after etching in aqueous TMAOH at 30-40℃ for 72 h. (d) Typical bright-field TEM images of the delaminated Sc-containing 2D flexible flakes, depicting scrolled behavior. Inset in (d) is a SAED pattern confirming the hexagonal basal plane symmetry.
X-ray photoelectron spectroscopy (XPS) was used to gain more information on the etching paths and resulting surface functionalization (detailed information can be found in Figure S4 and Table S2 in SI). After etching, the sample exhibited an additional signal corresponding to N 1s at about 402.01 eV (Figure 2), thus verifying intercalation of TMA+ ions between the sheets.37 The Sc 2p and O 1s spectra before and after TMAOH treatment are shown in Figures 2a and 2b, respectively. The presence of Sc-C and Sc-Ox bonds before treatment is similar to previous works on binary ScC.39 Due to the high affinity between Sc and O, spatial distribution of element O was observed being concentrated at the crystal edges of ScAl3C3 crystallites (Figures S2c-f), implying the presence of a thin oxidized layer on the crystal surfaces, which is possibly the main source of the relatively large amount of Sc-Ox bonds. After TMAOH etching, the Sc 2p region exhibits Sc-C,39 mixed Sc-C [with OH or Al(OH)4- surface groups],40 and Sc-Ox bonds [Al(OH)4- group or thin oxidized surface layer].41 The corresponding fitted results in the O 1s region confirm 8 ACS Paragon Plus Environment
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the presence of Sc-Ox,41 and Al-O (OH groups)42 in the original ScAl3C3 powders. Moreover, oxygen-related species shifted to higher energies for the fitted results of the etched sample, mixed Sc-C-(OH or Al(OH)4-)37 and partially oxidized Sc-Ox41 bonds are identified, suggesting the formation of ScCx with a mixture of OH, Al(OH)4- surface groups and intercalated TMA+ ions. It is noted that the Sc-Ox bonds after the TMAOH treatment were mainly come from the Al(OH)4- surface group and inevitable thin surface oxidized layer. An apparent loss of Al was observed after etching, in contrast to previously reported TMAOH-treated Ti3AlC2.37 According to the fitted XPS results in the C 1s region, the relative content of Al-C bonding decreased from 25.4% to 1.09% after TMAOH treatment (Figures S4a-b and Table S2), but the relative content of Sc-C bonds shown a minor decrease, which suggests that the relatively weakly bonded and easily hydrolyzed Al-C sub-layer of the original ScAl3C3 has reacted with TMAOH and hydrolyzed into Al(OH)4-, followed by a partial replacement of Al(OH)4- by OH groups in aqueous solution. Since the crystal structure of the ScAl3C3 can be described as an intergrowth structure with hexagonal Sc-C layer and Al3C2 sharing a carbon monolayer as their coupling boundaries, the carbon vacancies can be formed in the reserved ScC slabs during the etching process, thus the formula of ScCxOH is adopted to present this "MXene" synthesized.
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Figure 2. Surface functionalization and band gap of 2D ScCxOH. High-resolution XPS spectra of samples before and after etching treatment in the (a) Sc 2p, N 1s, and (b) O 1s region, respectively. (c) Sc L3,2, O K-edges, and Al K-edges, collected from the same region after first (top) and second (bottom) measurement. Sc L3,2 edge shows a significant redshift and Al K-edge loses most of its intensity in the second measurement. (d) Monochromated STEM-EELS spectrum of delaminated ScCxOH sheets, right inset is corresponding STEM image.
The flakes were further investigated by scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS). Based on the structural appearance of the particles as presented in Figure 1d, a number of similar particles were identified, and the composition was investigated with EELS. It was 10 ACS Paragon Plus Environment
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found that the composition varies significantly in between scans, even within single particles, particularly for Sc, Al and O. A key to this apparent discrepancy may be found when examining the two EEL spectra presented in Figure 2c. The spectra identify the Sc-L3,2, C-S, O-K and Al-K edges for two sequential acquisitions on the same particle, in the same area. The accurate content of C was not discussed here in consideration of potentially partial contribution from contamination. The initial spectrum exhibits pronounced edges for all three elements with the relative composition 1.00:2.88:1.38 for Sc, O and Al respectively. After the 2nd acquisition, the relative composition has changed to 1.00:1.94:0.29. This identifies a distinct loss of Al(OH)4- surface group. For reference, images of the investigated region show the same structural appearance prior to both acquisitions (see Figures S4c-d). This information is significant as it identifies the Al(OH)4- group to be a volatile species on the ScCx surface that desorbs upon local heating caused by the electron beam. The functionalizing groups on MXenes were previously found to be both mobile and volatile.43 F was additionally identified by EELS, which may introduce by raw scandium powders due to the production process, and it was observed to a negligible amount but desorbed between acquisitions. This is not surprising since F is known to be a volatile species as observed during previous in situ heating experiments.44 N was not identified as the N-K edge (~401 eV loss) entirely overlaps the Sc-L3,2 edge (~402 eV loss). In addition to the elemental variations observed in-between scans, an apparent chemical shift occurs for the Sc-L3,2 edge, which redshifts by ~4 eV. There is also a 11 ACS Paragon Plus Environment
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slight difference in the edge fine structure (the L3 and L2 peaks are distinctly separated in the 2nd spectrum). For the Al-K edge, the noise level is too significant to allow for a determination of chemical shifts. The O-K edge, on the other hand, does not experience a chemical shift. However, the edge peak is redshifted by ~4 eV. This apparent discrepancy most likely originates from the overlap of two O-K signatures, where one component does not experience a (significant) shift, while the other component is desorbed and causes the loss of O and a redistribution of the peak intensity. The strong chemical shift associated with the Sc edge, identifies the desorbing Al(OH)4- group to be chemically bound to (functionalizing) the Sc surface prior to desorption. The chemical shift is caused by a change in the Fermi energy associated with the redistribution of electrons as the functional groups desorbed and have been observed in O-functionalized Ti3C2 MXene previously.43 For a spatially selective measurement of the optical properties, monochromated STEM-EELS was employed, and the results are shown in Figure 2d. A delaminated ScCxOH flake was selected (inset in Figure 2d) and the valence EELS (VEELS) properties of the flake were acquired as an average from this flake. The energy resolution was 80 meV as can be seen in Figure 2d, which is significant for precise optical measurements using EELS. Generally, a semiconducting material starts to absorb energy from the electron beam since the transition from the valance band to the conduction band, and a sharp onset occurs at an energy loss of band gap Eg.45,46 Because of the 2D nature of the flake, the signal is low. After a power-law background subtraction, the obtained valence spectrum was magnified by 25kx and 12 ACS Paragon Plus Environment
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identifies the flake to exhibit a bandgap. This is shown as a rapid increase in the VEELS response at ~2.55 eV. The bandgap, and its uniformity, is further supported by cathodoluminescence measurements (see Figure S6). Having established that the as-etched 2D scandium carbide is a semiconductor with a bandgap of about 2.5 eV, we proceed to determine its photoresponse in the ultraviolet-visible light region (Figure 3). The photocurrent of the device made from delaminated ScCxOH (d-ScCxOH) few layers was characterized by a two-point probe station in air. A pair of Ti/Au (10 nm/80 nm) electrodes were deposited on the d-ScCxOH flakes by using standard photolithography. Figure 3a illustrates the fabricated photodetecting device, and the inset is a digital photo of the d-ScCxOH dilute solution that was used for device fabrication. Figure 3b shows the current-voltage (I-V) characteristics of the device illuminated with light at different wavelengths and at dark condition, respectively. The photocurrent shows only slight changes for wavelengths exceeding 550 nm. At shorter wavelengths, marked photocurrent is observed. At 360 nm light with a power density of 160 μw/cm2, the ratio of photocurrent/dark current is around 100 times, indicating the dark current is at the noise level (
