Direct Synthesis and Practical Bandgap Estimation of Multilayer

Jan 7, 2016 - Black phosphorene, which is a single-atomic layer of black phosphorus, was first widely investigated because its bandgap can be increase...
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Direct Synthesis and Practical Bandgap Estimation of Multilayer Arsenene Nanoribbons Hsu-Sheng Tsai,*,† Sheng-Wen Wang,‡ Ching-Hung Hsiao,§ Chia-Wei Chen,§ Hao Ouyang,§ Yu-Lun Chueh,§ Hao-Chung Kuo,‡ and Jenq-Horng Liang*,†,∥ †

Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 30013, Taiwan, R.O.C. Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30013, Taiwan, R.O.C § Department of Material Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, R.O.C. ∥ Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan, R.O.C. ‡

S Supporting Information *

biaxial tensile strain on band structure of arsenene. The ∼2.49 eV-indirect bandgap is gradually reduced with the biaxial tensile strain. Moreover, the indirect−direct bandgap transition occurs, as the biaxial tensile strain is equal to 4%. Zhu et al.13 used the Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional for most calculations to predict the variation of band structure of gray arsenene caused by a strain and stacking sequence. C. Kamal et al.14 utilized the QUANTUM ESPRESSO package to perform fully self-consistent density functional theory (DFT) calculations for simulation of band structures of the three geometric arsenene layers under different strain states. As mentioned above, the research progress of arsenene is still under a theoretically computational stage so far. We have developed a mature technique, so-called plasmaassisted process, to synthesize multilayer graphene on 4H-SiC and demonstrated its significant electronic properties.15 This technique was further expanded later and synthesized the multilayer germanene on SiGe/Si as well as multilayer violet phosphorene on InP.16,17 In this work, the multilayer arsenene was successfully synthesized on InAs using the plasma-assisted process. The formation mechanism of multilayer arsenene would be interpreted in detail. We found that the multilayer arsenene obtained by our process is actually not continuous, implying that it is like a pile of multilayer nanoribbons. Most importantly, we estimated the bandgap of the multilayer arsenene nanoribbons by the PL measurement, indicating that it possesses great potential to be applied to switching and light-emitting devices. On the basis of the concept of the plasma-assisted process, the nitrogen ions introduced into InAs would preferentially react with indium and simultaneously squeeze arsenic atoms out of the surface to form the arsenene layers during the thermal treatment. The synthesis process was optimized by tuning experimental conditions including annealing time, power of plasma and exposure time of plasma. The isothermal annealing was first implemented for various annealing time after N2 plasma immersion. According to our previous study,17 the annealing temperature should be fixed at 450 °C. Raman spectra of the samples after process with different annealing times are shown in Figure 1a. The transverse optical (TO) and longitudinal optical

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ecause of the zero-bandgap of group IV 2D materials such as graphene, silicene, and germanene,1,2 it totally restricts their applications for channel and emission materials in switching and light-emitting devices respectively, even though the bandgap can be opened by applying a vertical electric field3,4 or by introducing particular species.5−7 Hence, the 2D materials of group V elements have begun to be noted recently. Black phosphorene, which is a single-atomic layer of black phosphorus, was first widely investigated because its bandgap can be increased from ∼0.34 to ∼2 eV once the single layer is reached.8 In addition, it possesses significant electronic properties including high carrier mobility (∼1000 cm−1/(V s)) and on−off current ratio (>105).9 However, black phosphorus is quite unstable under atmospheric environments and the synthesis conditions are very difficult to achieve. Apart from the exfoliated black phosphorus nanosheets,9 whose area and thickness are uncontrollable, Li et al.8 synthesized large scale black phosphorus using a cubic-anviltype apparatus to directly heat the powder of the red phosphorus on a SiO2/Si substrate at a high temperature (1000 °C) under relatively high pressure (10 kbar). According to their angleresolved photoemission spectroscopy (ARPES) and ab initio band structure calculation results, the black phosphorus obtained from this method is multilayer, implying that it is not proper to be used as the channel material of transistors. In their study, the black phosphorus nanosheets mechanically exfoliated by the Scotch tapes were still used for the fabrication of field-effect transistors (FET). On the other hand, T. Nilges et al.10 replaced a high pressure chamber with a sealed silica ampule (length 50 mm, inner diameter 8 mm) containing the solid catalyst (SnI4) with a low boiling point (∼364 °C) in order to induce the transformation from the red phosphorus to black. Even so, the scale of the black phosphorus would be limited and the residual catalyst could not be completely removed for future applications. Therefore, the real applications of the black phosphorus are controversial up to now. Afterward, the arsenene, which is a single-atomic layer of gray arsenic with a rhombohedral structure,11 attracted researchers in the field of 2D materials as well. Semimetallic gray arsenic bulk constructed by buckled honeycomb arsenic layers is the most stable phase among all of the arsenic allotropes.11 It is worth noting that the bandgap of gray arsenic could be changed from ∼0.175 to ∼2.49 eV as the scale is down to a monolayer (arsenene).11,12 S. Zhang et al.12 further computed the effect of © XXXX American Chemical Society

Received: December 22, 2015 Revised: January 6, 2016

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DOI: 10.1021/acs.chemmater.5b04949 Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials

Figure 1. (a) Spectra of intrinsic InAs and InAs after N2 plasma immersion with the power of 200 W for 30 min followed by annealing at 450 °C for different periods. (b) Spectra of intrinsic InAs and InAs after N2 plasma immersion with different powers for 30 min followed by annealing at 450 °C for 30 min. (c) Spectra of InAs after N2 plasma immersion with the power of 100 W for different periods followed by annealing at 450 °C for 30 min.

(LO) modes of InAs, merged into a broad peak, are located at 217.3 and 238.6 cm−1 respectively.11 Obviously, the optimized annealing time is 30 min because the phonon mode of arsenene, which is slightly shifted from the computational peak position,12 emerges at ∼259 cm−1 in the spectrum. The cause of this peak shift might be the strain effect as we mentioned in the previous reports.15,16 As the annealing time reduces from 30 to 10 min, the thermal energy is too low to achieve the formation of arsenene layers and the recovery of damaged InAs surface resulting from the ion bombardment of plasma. Hence, there are only quite weak Raman peaks of InAs in the spectrum. Conversely, the much longer annealing time provides sufficient energy to synthesize arsenene layers, but it becomes much thicker, like bulk gray arsenic due to the excess squeeze. As can be seen, the Eg (195 cm−1) and A1g (257 cm−1) peaks of gray arsenic11 can be observed in the spectrum of the sample after plasma immersion and annealing at 450 °C for 3 h. Thus, the annealing time would be fixed at 30 min. The power of N2 plasma was next optimized by the exposure of InAs samples with varying powers of 50 to 200 W in 50 W steps for 30 min. Raman spectra of the samples after process with different powers of plasma are shown in Figure 1b. Apparently, the arsenene layers could be synthesized no matter what power of plasma we set. The N2 plasma with power of 100 W would be selected for the synthesis owing to the stronger phonon mode of arsenene than those of the InAs substrate, implying that the crystallinity of arsenene layers is good enough. Although the higher power of plasma causes a higher intensity of the phonon mode of arsenene, it is not selected in our process because more defects might be created by increasing the power. Finally, the exposure time of N2 plasma was also changed for optimization of the process. Figure 1c exhibits that the Eg peak of gray arsenic arises and becomes more intense by extending the exposure time. It means that the thickness of gray arsenic obtained by the plasma-assisted process could be controlled by altering the exposure time. The nitrogen concentration near the surface of InAs increases with the exposure time of plasma due to the ion accumulation. The concentration profile is broadened via the internal ion diffusion during annealing. Therefore, the amount of arsenic atoms squeezed onto the surface becomes more, resulting in thicker gray arsenic. After a series of experiments, the plasmaassisted process for synthesis of arsenene layers would start from the N2 plasma immersion with the power of 100 W for 30 min followed by annealing at 450 °C for 30 min. The chemical configuration of elements at the surface was analyzed by X-ray photoemission spectrometer (XPS). The In− N peak splits into 3d5/2 and 3d3/2 bands, located at ∼443.1 and ∼450.6 eV respectively, in the In 3d spectrum of Figure 2a. It

Figure 2. (a) In 3d spectrm of the multilayer arsenene/InN/InAs. (b) As 3d spectrum of the multilayer arsenene/InN/InAs.

indicates that the indium at the surface of InAs has reacted with the nitrogen ions introduced by plasma. The inevitable As−O signal (∼43.9 eV) caused by the atmosphere is detected in the As 3d spectrum of Figure 2b. It is noted that the As−As peak located at ∼42.4 eV infers the formation of arsenic of the elemental structure. According to the XPS results mentioned above, it seems that the InN that is formed during annealing might squeeze arsenic atoms out of the surface to rearrange them into arsenene layers. This is consistent with our anticipation and more analyses would be implemented for further identification of material. The cross-sectional layer structure at nanoscale was monitored by transmission electron microscopy (TEM). As shown in Figure 3a, the SiO2 capped on the surface was deposited by an electron beam evaporator to protect the thin film from the damage caused by the focus ion beam (FIB) during the TEM sample preparation. Under high magnification, it is clear to observe the heterogeneous structure that consists of three parts including multilayer arsenene, InN, and InAs substrate. Reasonably, the (110) interplanar distance (∼0.43 nm) of InAs in the TEM image is consistent with that (∼0.428 nm) of the theoretical atomic model as shown in Figure 3b. The InN layer, which originates from the reaction between nitrogen ions and indium in InAs, is highly strained because it is like a buffer layer to match the lattice of multilayer arsenene with that of InAs. It is necessary to verify that the top layer of the TEM image is multilayer arsenene indeed. The diffraction pattern in the inset of Figure 3a is processed by fast Fourier transform (FFT) in order to derive the interplanar distances of multilayer arsenene. Two groups of reciprocal lattice points are chosen for computation of the particular interplanar distances of plane groups. These two calculated interplanar distances, which are equal to ∼0.286 and ∼0.181 nm (Figure S1), correspond to the (110) and (01-1) interplanar distances (Figure 3b) of rhombohedral gray arsenic, respectively. Moreover, the angle between the two lines representing the (110) and (01-1) plane groups in the diffraction B

DOI: 10.1021/acs.chemmater.5b04949 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 3. (a) TEM image of the multilayer arsenene/InN/InAs. (b) Theoretical atomic model of multilayer arsenene/InN/InAs layer structure. Inset: diffraction pattern of multilayer arsenene.

Beyond synthesis of multilayer arsenene, the bandgap of multilayer arsenene was estimated by measuring the PL characteristic. Before the sample was loaded onto the stage in the chamber, the spectrum shown in Figure 5 was first taken as

pattern is nearly equal to that between the (110) and (01-1) plane groups in the real lattice (Figure 3b). Thus, the top layer could be identified as the multilayer arsenene. The variation of concentration depth profile during annealing was detected by a secondary ion mass spectrometer (SIMS). After plasma immersion, the nitrogen ions initially congregate from the surface to the depth of ∼20 nm as shown in Figure 4.

Figure 5. PL spectra of the multilayer arsenene/InN/InAs under different temperatures. Inset: photograph of the sample in the chamber of spectrometer. Figure 4. SIMS depth profile of arsenic and nitrogen elements. Inset: schematic layer structure.

the reference. Then the temperature was reduced in order to restrain the trapping of defect states and a series of measurements under different temperatures began to be implemented. As can be seen in Figure 5, there is a quite weak PL peak that emerges at ∼540 nm in the spectrum under 150 K. Its intensity increases to triple by reducing the temperature from 150 to 105 K. As 90 K is reached, the PL intensity greatly increases to ∼6 × 105 corresponding to the photograph that shows the intense green light emitted from the multilayer arsenene in the inset of Figure 5. The InN layer and InAs substrate should be excluded from the contribution of the green light emission because their bandgaps are respectively 0.65 and 0.354 eV, which are not within the range of energy of the spectrum. The green light (540 nm) emission indicates that the bandgap of the multilayer arsenene obtained in this work is ∼2.3 eV. Paradoxically, the bandgap of arsenene layers can only be opened as the number of layers is below 2 according to the computational result.12 However, the multilayer arsenene obtained in this study should not be bilayer or monolayer because its thickness is ∼14 nm. There is a dual factor resulting in the bandgap opening of the multilayer arsenene. One is the quantum confinement effect;18 the other is the turbostratic

After the thermal treatment, the nitrogen ions further diffuse into the substrate at the depth of ∼40 nm. On the other hand, the intensity of arsenic concentration near the surface increases from ∼6 × 103 to ∼2 × 104. It is implied that the arsenic atoms are squeezed forward to the surface due to the formation of InN during annealing. The SIMS result confirms the formation mechanism of multilayer arsenene once again. All of the analytical results verify that the indium at the surface of InAs has preferentially reacted with the nitrogen ions from plasma to form InN during annealing, simultaneously; the arsenic atoms would be squeezed out of the surface to rearrange themselves into multilayer arsenene. Furthermore, the priority of occurrence between two chemical reactions depends on the ΔG values of them.15−17 In this case, the ΔG of InN formation is much more negative than that of AsN formation under the particular condition. Therefore, the former is a spontaneous reaction and the latter is not, resulting in the arsenic condensation on the surface. C

DOI: 10.1021/acs.chemmater.5b04949 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials stacking including kinds of stacking types such as AA stacking.13 It is noted that the as-synthesized multilayer arsenene is defective and noncontinuous like a pile of multilayer nanoribbons uniformly distributed on the substrate as shown in Figure 3a. The arsenic atoms are squeezed out from the damaged InAs caused by ion bombardment through the formation of InN to rearrange into multilayer arsenene on the surface so that the layer stacking should not be totally normal ABC stacking and the boundaries between nanoribbons should exist.15 In fact, the widths of multilayer arsenene nanoribbons are within a particular range. The TEM image of the region with wider multilayer arsenene nanoribbons chosen as Figure 3a is proper for analysis of crystal structure. The TEM image of the region with narrower ones for illustration of the nanoribbon structure is shown in Figure S2. Therefore, the turbostratically stacked nanoribbon structure with quantum confinement effect induces the bandgap opening of the multilayer arsenene. The intense PL peak at ∼2.3 eV leads it to have much potential for applications of switching and light-emitting devices. Nevertheless, there are not any PL peaks that could be detected from the thicker (∼21.4 nm) multilayer arsenene nanoribbons (not shown here), indicating that the transformation from semiconducting to metallic would occur from the thickness of ∼14 nm to that of ∼21.4 nm. It is worth noting that the 2D materials stacked by van der Waals force with the characteristic of green light emission have not been actually synthesized so far in addition to the multilayer arsenene nanoribbons in the work. In conclusion, the multilayer arsenene nanoribbons have been successfully synthesized on an InAs substrate using the plasmaassisted process. Its thickness could be changed by controlling the plasma exposure time. The formation mechanism is clearly interpreted by the results of material analysis. The arsenic condensation is induced by the preferential occurrence of indium nitridation due to large difference between the variations of ΔG of reactions. The bandgap of multilayer arsenene nanoribbons is estimated at ∼2.3 eV. This low-cost process has been utilized to synthesize four kinds of 2D materials, implying that it has vast potential for commercialization in industry.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge National Nano Device Laboratories for the SIMS analysis.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04949. Experiments and calculations (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*H.-S. Tsai. E-mail: [email protected]. *J.-H. Liang. E-mail: [email protected]. Author Contributions

H.-S.T. created the idea, optimized the plasma-assisted process, implemented the Raman analysis, processed the data, finished the calculations, and wrote the paper. S.-W.W. and H.-C.K. contributed the PL measurements. C.-H.H. and H.O. contributed the TEM analysis. C.-W.C. and Y.-L.C. contributed the XPS analysis. J.-H.L. supervised the project and contributed suggestions for the experiments. Funding

Financial support was provided by the Ministry of Science and Technology through Grant No. 104-2221-E-007-104-MY3. D

DOI: 10.1021/acs.chemmater.5b04949 Chem. Mater. XXXX, XXX, XXX−XXX