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Synthesis and Photovoltaic Properties of Cd3As2 Faceted Nanoplates and Nano-Octahedrons Cai-Zhen Li,†,# Rui Zhu,‡,# Xiaoxing Ke,§ Jing-Min Zhang,‡ Li-Xian Wang,† Liang Zhang,† Zhi-Min Liao,*,†,∥ and Da-Peng Yu†,‡,∥ †

State Key Laboratory for Mesoscopic Physics, Department of Physics and ‡Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871, P. R. China § Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, 100 Ping Le Yuan, Beijing 100124, P. R. China ∥ Collaborative Innovation Center of Quantum Matter, Beijing, P. R. China S Supporting Information *

ABSTRACT: Recent theoretical predictions and angleresolved photoemission spectroscopy measurements have shown that single crystal Cd3As2 is a three-dimensional topological Dirac semimetal possessing linear dispersions along all three momentum directions. Nanoscale topological Dirac semimetal structures have a large surface-to-volume ratio and provide a platform to explore its topological surface states. Here we report the synthesis of high quality Cd3As2 single crystalline nanoplates and nano-octahedrons via a vapor−solid growth mechanism. Triangular and hexagonal nanoplates with lateral dimensions ranging from several hundred nanometers to tens of micrometers are obtained. The top facets are (112), consistent with the natural cleavage plane of Cd3As2 single crystal. The synthesized Cd3As2 nano-octahedrons are enclosed by the {112} facets. A photovoltaic effect is demonstrated from a Cd3As2 nanoplate/metal electrode interface, suggesting potential applications in self-powered photodetection.

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high as 1.5 × 104 cm2/(V s) at room temperature and 8 × 104 cm2/(V s) at 4 K,14,15 promising for potential applications in electronic devices. Second, Cd3As2 is chemically stable in air atmosphere compared to other candidates, which allows easy measurement of its physical properties.6,8 Third, tetragonal Cd3As2 has C4 symmetry, different from graphene and other topological insulators, and is therefore a symmetry-protected semimetal. While much progress has been obtained from Cd3As2 bulk crystal via ARPES and transport measurements,16−19 the study of Cd3As2 nanostructures has just started. Omari et. al.20 reported the fabrication of Cd3As2 nanowires via vapor−solid growth mechanism without catalysts, and the infrared optical absorption measurements suggested the several optical absorption transitions of the nanowires, which makes it attractive for photodetectors and optoelectronic devices. Recently, Schönherr et al.21 presented a detailed synthesis of Cd3As2 nanowires. Besides, some recent reports reveal the magnetotransport properties of Cd3As2 nanostructures including negative magnetoresistance induced by chiral anomaly22 and Shubnikov-de Haas oscillations.23 As we known, topological nanostructures have the advantages of

INTRODUCTION Three-dimensional (3D) topological Dirac semimetal is a new quantum phase that has attracted much attention recently.1,2 3D Dirac semimetals and Weyl semimetals3,4 have linear energy-momentum dispersion relations along all momentum directions and can be considered as 3D graphene.5 For 3D Dirac semimetals, the conduction band and the valence band are connected at isolated points in momentum space (i.e., the Dirac points). Recently, such Dirac semimetals are experimentally achieved in Na3Bi6−8 and Cd3As29−11 bulk crystals synthesized by the flux method. The success has shed light on the study of exotic and rich physics in topological semimetals. Cd3As2 is a II−V group compound with an extremely high carrier mobility. Recently, it was identified to be a 3D Dirac semimetal containing two Dirac nodes along kz directions and linear dispersions along all momentum directions as confirmed by angle-resolved photoemission spectroscopy (ARPES)9−11 and scanning tunneling microscopy.12 In addition, He et al. have observed linear magnetoresistance and quantum oscillations at low temperatures and strong magnetic field through quantum transport measurements.13 The results suggest the existence of Berry’s phase π and provide evidence for a 3D Dirac semimetal phase in bulk Cd3As2. Among a number of Dirac semimetal materials, Cd3As2 has its own advantages and unique properties.9 First, the carrier mobility of Cd3As2 is as © XXXX American Chemical Society

Received: March 23, 2015 Revised: May 19, 2015

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easier modulation and significant size effect.24,25 In addition, Cd3As2 is also a potential material for photodetection and optical switches except for the exotic transport properties. It is therefore of great importance to synthesize Cd3As2 nanostructures with various morphologies. However, Cd3As2 polygonal nanostructures with regular shape have not been reported yet. In this work, we report the synthesis of Cd 3 As 2 nanostructures with various morphologies, including triangular, hexagonal, octahedral, and starlike shapes by the physical vapor deposition (PVD) method. The as-synthesized nanostructures exhibit high crystallinity, where exposed surfaces are determined as {112} facets. Density functional theory (DFT) calculations of free surface energies are performed, which indicates a possible growth mechanism. The photovoltaic effect is further demonstrated at the Cd3As2/metal electrode interface, which suggests its potential applications in optoelectronic devices.

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RESULTS AND DISCUSSION Figure 1a shows a typical TEM image of a triangular nanoplate in low magnification. The edge length of this triangular is about

EXPERIMENTAL AND CALCULATION METHODS

Nanostructure Synthesis. The Cd3As2 nanostructures were synthesized by the PVD method in a horizontal furnace.25−29 The PVD equipment is placed in a ventilation closet and the pumper is equipped with a cold trap to collect the dust in the exhaust. The Cd3As2 powders (purity >99.99%, Alfa Aesar) were placed at the center of the tube furnace, while silicon substrates were placed downstream of the source to collect the products. We first flushed the tube by Argon three times to get rid of oxygen. The temperature of the furnace was then increased from room temperature to 650 °C in 20 min and kept for 10 min for growth. During the growth process, the pressure in the PVD furnace was kept at about 1 atm, and the carrier gas (argon) was maintained at 20 sccm. After deposition, the system was cooled down from 650 to 350 °C at the rate of 5 °C per minute and then cooled down to room temperature naturally. The nanostructures were collected on substrates located 14−17 cm away from the Cd3As2 source, where the temperature was estimated to be about 300 °C during growth. Characterization. The scanning electron microscope (SEM) characterizations were performed on a FEI NanoSEM 430 system. The Cd3As2 nanostructures were transferred to a copper grid coated with an ultrathin carbon film for preparing the transmission electron microscope (TEM) sample. The TEM characterizations, selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDS) were performed on a FEI Tecnai F20 TEM. Device Fabrication and Photoelectric Measurements. The asgrown Cd3As2 nanoplates were first transferred onto a silicon substrate topped with a SiO2 layer of 285 nm. Then Ti/Au pads were deposited near the Cd3As2 nanoplate using electron beam lithography. Pt electrodes were fabricated by focused ion beam induced deposition to bridge the nanoplate and the Ti/Au pads. The electrical signals were measured using Agilent 2912A Source-Meter Units. The laser illumination was guided by a Renishaw confocal optical system. DFT Calculations. The DFT calculations were performed with the CASTEP code using ultrasoft pseudopotentials and exchangecorrelation function of general gradient approximation (GGA) of PBE.30,31 Total energies of the surface structures were obtained by total energy calculations with geometry optimization. The plane-wave cutoff energy was set to be 240 eV, and 1 × 1 × 1 Monkhorst−Pack kpoint sampling was chosen. Cd3As2 unit cell structures with lattice parameters of a = b = 12.633 Å, and c = 25.427 Å were used to construct the structures.5 In geometry optimization, the force convergence threshold on each atom was set to be less than 0.1 eV/ Å. Total energy of Cd3As2 unit cell with 96 Cd atoms and 64 As atoms was first calculated to get the chemical potential (μ) of Cd3As2 unit in bulk. Then, specific surface free energies can be formulated following σ(hkl) = [Esurf − Nμ]/2A, where Esurf, N, and A are the DFT total energy, number of Cd3As2 units, and the surface area of the constructed (hkl) plane, respectively.

Figure 1. TEM images of Cd3As2 nanostructures. TEM image (a) of a typical triangular nanoplate and its magnification is shown in (b); (c) the HRTEM image of the area marked in (b). The inset in (c) shows the corresponding diffraction pattern from triangular nanoplate along the [221] zone axis. (d) TEM image of a typical octahedron, and (e) shows the enlarged TEM image of its edge. (f) is the corresponding HRTEM image from the indicated area in (e). The inset in (f) is the corresponding FFT pattern of the [001] zone axis.

500 nm. High-resolution TEM (HRTEM) images of the triangular nanoplate (Figure 1b,c) show a lattice spacing of 0.225 nm, corresponding to the (048̅ ) plane spacing. SAED pattern for the triangular nanoplate clearly shows hexagonal symmetry of the (112) projection, as shown in the inset of Figure 1c. Similarly, Figure 1d is a typical TEM image of an octahedron at low magnification, and the edge length of this octahedron is about 150 nm. HRTEM images of the octahedral in Figure 1e,f show a lattice spacing of 0.32 nm, and the corresponding fast Fourier transformation (FFT) as shown in the inset in Figure 1f can be indexed to the [001] zone axis. We have tested tens of samples by HRTEM and confirmed the same crystal orientation of the nanostructures. Figure 2a shows the EDS of a Cd3As2 triangular nanostructure. The Cd and As peaks can be clearly identified and are further quantified to have an atomic ratio of nearly 3:2 as expected. The peak of Cu resulted from the copper grid used for carrying the sample. To further confirm the crystal structure, X-ray diffraction pattern of Cd3As2 nanostructures is exhibited in Figure 2b. It demonstrates the tetragonal crystal structure despite some small peaks resulting from the other sediments on the substrate. In addition, the spectra of X-ray photoelectron spectroscopy B

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Figure 2. (a) EDS of a Cd3As2 triangular nanoplate. (b) XRD pattern of Cd3As2 nanostructures. Panels (c) and (d) are XPS spectra of the Cd3As2 structures, showing the Cd 3d and As 3d peaks.

(XPS), shown in Figure 2c,d, indicate both Cd 3d peaks and As 3d peaks, which confirms the right elemental composition of the synthesized Cd3As2 nanostructures. The crystal structure of Cd3As2 is body-centered tetragonal with a large unit cell (a = b = 12.633 Å, c = 25.427 Å) and belongs to the space group of I41/acd,5 as illustrated in Figure 3a. The large unit cell contains 2 × 2 × 4 reduced cells of pseudo cubic structure, as shown by the schematic diagram in Figure 3b. In the pseudo cubic cell, the As atoms are arranged in a fcc configuration, and each As atom is coordinated to six Cd atoms in a tetrahedral arrangement. Triangular and octahedral nanostructures are the most typical structures among all sample morphologies, as shown in Figure 3c,d, respectively. The triangular nanoplate has a lateral dimension of ∼4 μm and a thickness of ∼1 μm as measured by atomic force microscopy (AFM, Figure S1). More SEM images of the triangular nanoplates and the nano-octahedrons are presented in Figure S2. The top surface of the triangular nanoplate can be identified to be (112). Its side length is nearly equal, and the (112̅), (11̅2), (1̅12) side facets have an intersecting angle of 60°. An octahedron along the [001] axis with ending surfaces of {112} is shown in Figure 3d. The (112) surface is the natural cleavage surface with a relatively large lattice spacing of 0.73 nm. Meanwhile, the surface free energy of the (112) facet is smaller than that of other facets, which makes it the most favorable grown facet. Here we take the triangular nanoplate as an example to show how to assign the crystal facets. According to the TEM results, the zone axis of the triangular nanoplate is [221], which corresponds to (112) top surface plane. As it is viewed along the [221] zone axis, there are nearly equal lateral sides marked by yellow arrows shown in Figure 4a. We can tilt the sample stage to a specific angle (∼19.5 deg) at which one of the side facets happens to disappear totally in view, and the corresponding zone axes and lattice planes are [111], (112̅); [241], (11̅2); [421], (1̅12) respectively. Here we take the [111]

Figure 3. Crystal structure and SEM images of Cd3As2 nanostructures. (a) Cd3As2 large unit cell in tetragonal structure with lattice constants of a = b = 12.633 Å, c = 25.427 Å. (b) The pseudo-cubic cell structure. (c) SEM image of a typical triangular nanoplate. (d) SEM image of a typical octahedron nanostructure with the [001] zone direction and {112} surfaces.

zone axis and its disappeared (112̅) plane as example to demonstrate the corresponding relation between experimental results and structure model. As shown in Figure 4b, we tilt the sample stage to 19.5 deg at which the upper side facet exactly disappears from view. In simulation, the structure viewed from the [221] zone axis is illustrated in Figure 4c, which has pseudohexagonal symmetry. Then, we rotate the lattice array by C

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Figure 5. Method of indexing side facets of the nano-octahedron. (a) SEM image of an octahedron viewed from the [221] zone axis and (b) tilting 20 deg of sample stage when the upper side facet exactly disappears from view. Panels (c) and (d) are atomic structures viewed from the [221] and [111] zone axes, respectively. Panel (e) is the cross section of the octahedron along the red dashed line marked in (a), and m is the side length of the octahedron. For an octahedron, the inclined 1/2

angle between zone and side-facet θ′ = arcsin[1/2m/(3 /2)m] = 35.26°, then the inclined angle between the neighboring side facets can be calculated to 109.48°, consistent with the inclined angle between the (112) and (112̅) planes and tilt angle in the SEM experiments. Note that along the [111] zone axis one obtains the side view of the (112̅) plane. The (112) plane and (112̅) plane, as well as other equivalent side facets, are in the same family of crystal planes. Therefore, the octahedron is closed by {112} facets.

Figure 4. Method of indexing side facets of the nanoplates. (a) SEM image of a triangular nanoplate viewed from the [221] zone axis and (b) tilting 19.5 deg of sample stage when the upper side facet exactly disappears from view. The white circles in (a) and (b) are zone axes and white arrows are crystal orientations. Panels (c) and (d) are atomic structures viewed along the [221] and [111] zone axes, corresponding to top view of the (112) plane and side view of the (112̅) plane, respectively. Panel (e) is the cross section of triangular nanoplate along the red dashed line marked in (a). (f) Pattern for calculating angle between the [221] and [111] directions; a and c are crystal lattice constants, a = 12.633 Å, c = 25.427 Å.

19.5 deg at which one can clearly see the side view of the (112)̅ plane from the [111] zone axis, shown in Figure 4d. The disappeared side facet should be parallel with the [111] axis, which forms a 19.5 deg angle with the [221] axis. Then we calculate the inclined angle between [111] and [221] directions within Cd3As2 structure model. In details, the cross section of the triangular nanoplate along the red dashed line (marked in Figure 4a) is schematically shown in Figure 4e, where the [221] and [111] zone axes with inclined angle θ are denoted clearly. Then we can calculate the inclined angle between [221] and [111] in Figure 4f; a and c are the lattice constants of Cd3As2 which are 12.633 and 25.427 Å, respectively. Our calculated angle θ′ = 19.47 deg is very consistent with the tilt angle in the SEM experiments. By the same method, the side facets of the octahedron can also be assigned (Figure 5). The Cd3As2 nanostructures with other morphologies such as hexagonal and starlike shapes were also obtained, as shown in Figure 6. The hexagonal nanoplates shown in Figure 6a,b have sizes up to ∼10 μm in lateral dimensions and hundreds of nanometers in thickness. The starlike nanostructures with five top surfaces are as shown in Figure 6c,d. To further explore the morphology distribution after a standard growth process, we have randomly selected an area of 0.2 × 0.2 mm2 on the substrate to count the number of each nanostructure. The results are shown in Figure S3, indicating a majority (62.3%) of triangular nanoplates, followed by nano-octahedrons (33.4%) and a minority (4.3%) of hexagonal nanoplates and few starlike nanostructures. It should be noted that the Cd 3 As 2

Figure 6. SEM images of Cd3As2 nanostructures with (a, b) hexagonal and (c, d) starlike morphologies.

nanostructures with regular morphologies can only be obtained at the substrates 14−17 cm away from the source, while at other places little products can be obtained. For comparison, Cd3As2 nanostructures synthesized under other conditions, that is, the temperature kept at 650 °C for 5 min for growth and the collection substrates placed at different positions, are shown in Figure S4. One can see that there were little products at the 13 cm position, while octahedral and triangular structures were obtained at 14−15 cm positions away from the source, but not D

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as smooth and regular as that synthesized for 10 min shown in Figure 3. Comparison of the products shown in Figures S2 and S4 indicates that the set of conditions, that is, the growth temperature kept at 650 °C for 10 min and the collection substrates placed away from the source about 15 cm, was the most optimal. The as-synthesized Cd3As2 nanostructures are very stable in atmosphere as testified from the thermogravimetric analysis shown in Figure S5. The well-known vapor−solid (VS) growth mechanism is used to account for the synthesis of our nanostructures. Cd3As2 precursors in the vapor phase are carried by Ar carrier gas toward the substrate and then nucleated. Epitaxial growth on the surfaces of the as-formed nuclei continues after the initial nucleation on the substrate. Figure 7 shows a schematic diagram of the suggested growth

Figure 7. Schematic diagram of the growth process.

processes. It could be proposed that the epitaxial growth process will enhance the as-formed structures without changing initial morphologies by considering the surface free energy of different facets (Figure 8). First, we estimated the surface free energies for facets of low Miller index by total energy DFT calculations, as shown in Table S1 in Supporting Information. The {112} facet has the smallest specific surface free energy of 0.0217 eV/Å2, while other facets such as {110}, {012}, and {100} have higher values of 0.0274 eV/Å2, 0.0284 eV/Å2, and 0.0569 eV/Å2, respectively. Second, the shape of Cd3As2 nucleus can be constructed following the Gibbs−Curie−Wulff (GCW) theorem,32 that is, σ(hkl)/d(hkl) = constant, where σ(hkl) is the surface free energy of (hkl) plane, and d(hkl) is distance from the center of the crystal to the surface plane. Figure S6 shows the ideal equilibrium shape, where the octahedron is enclosed by the {112} facets. The theoretical analysis agrees well with the experiment results, where all observed nanostructures are dominated by {112} exposed facets. Cd3As2 is demonstrated to have several infrared absorption transitions20 and the “threshold” wavelength is found to beyond 2 μm,33 which makes it a potential material for optoelectronic devices and can be used as photodetectors in the infrared region. Because of its electronic analogue to 3D graphene, we then demonstrate the photocurrent generation from the Cd3As2 nanoplate/metal interface. The inset in Figure 9a shows a SEM image of a typical device based on a single Cd3As2 nanoplate. The linear current−voltage (I−V) curve shown in Figure 9a indicates the ohmic contacts between the Cd3As2 nanoplate and the Pt electrodes. Because Cd3As2 is a 3D Dirac semimetal and can be viewed as 3D analogue of graphene, the properties of

Figure 8. Relaxed Cd3As2 structures with (a) (112), (b) (110), (c) (012), and (d) (100) surface for the DFT total energy calculations, respectively. The atoms enclosed in the dashed yellow box in each figure have fixed fractional coordinates. Inset: top view of each surface structure.

Cd3As2/metal interface should be very similar to that of the graphene/metal interface. It was reported that in graphene photodetectors the gapless nature of graphene allows for the almost unimpeded transmission of carriers through the potential barrier at the graphene/electrode interface and forming ohmic contact.34 Figure 9b shows the energy band diagram near the contact interface. The work function of the Pt is larger than that of Cd3As2, which results in a built-in electric field at the contact interface directing from the Cd3As2 nanoplate to Pt electrode. Similar to the graphene/metal interface, the photocurrent can be generated at the Cd3As2/ metal interface when illuminated. Figure 9c,d shows the time response of the open-circuit voltage (VOC) and short-circuit current (ISC) when different areas on the device were E

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Figure 9. Photoelectric response of individual Cd3As2 nanoplate. (a) I−V curve of the device. Inset: SEM image of the device. (b) Energy band diagram of the device. Ei refers to the built-in electric field at the interface between the electrode and Cd3As2. (c) The open-circuit voltage and (d) the short-circuit current with switching the illumination. The number marks correspond to the results measured at the positions marked in the SEM image in (a). (e) The open-circuit voltage and (f) the short-circuit current dependence with laser illumination intensity of point 4. The 100% laser power is ∼1.5 mW.

illuminated by a 514 nm laser (spot size ∼2 μm) at room temperature. The numbered curves in Figure 9c,d correspond to the measurement results from the numbered positions in the SEM image in Figure 9a. The output photovoltage was about 20 mV, and the generated photocurrent was about 320 nA when illuminating the interface between the electrode and the nanoplate. The photoelectric responsivity of the device was calculated by Iph/Pin, where Iph is the photocurrent and Pin is the effective incident optical power. The Pin was calculated by incident optical power density multiplying effective device active area. Considering that the total incident optical power was about 1.5 mW, the light spot was about 2 μm in diameter, and the effective device active area was about 1 μm × 300 nm (contact area of Pt and Cd3As2 nanoplate), the photoresponsivity of the Cd3As2 device was estimated to be 8.88

mA/W, which is larger than that of graphene photodetectors with 0.5 mA/W in the lateral structure34 and 6.1 mA/W in interdigitated electrodes structure.35 In order to rule out the possible thermoelectric effect, the photoresponses were measured when illuminating other areas. The point 2 marked in the SEM image in the inset of Figure 9a is off-centered on the sample and also away from the electrode/ Cd3As2 interface. If the thermoelectric effect is notable, the local heating by the laser illumination on point 2 would generate current in the circuit. However, in the actual experiments we did not detect any notable current when illuminating at point 2, so the thermoelectric effect can be neglected in our experiments. In addition, the photovoltage and photocurrent generated from the illumination on point 4 with the change of the laser intensity are shown in Figure 9e,f. The F

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open-circuit voltage and short-circuit current increase with the enhancement of the laser intensity. It is also found that the ISC deviates from linear dependence under high illumination intensity. Under the illumination with high laser intensity, the laser-induced heating cannot be neglected. Because of the linear energy-momentum dispersion relations, the carriers can also be thermally activated, which will increase the carrier concentration in Cd3As2 and thus reduce the contact potential between Cd3As2 and metal electrode. Therefore, under high illumination intensity, the ISC is smaller than that expected from the linear dependence. Compared with conventional semiconductor optoelectronic materials, Cd3As2 has linear dispersions near Dirac points and several band transitions, allowing potential applications in broadband photodetection and ultrafast optical devices. As analogues of graphene, Cd3As2 has similar electronic properties and can be viewed as 3D graphene, which may overcome the shortcoming of the low absorption rate (∼3%) of monolayer graphene. Therefore, along with the advantages of ultrahigh carrier mobility and high absorption rate, Cd3As2 may have potential applications in optoelectronics.

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CONCLUSION In summary, we have synthesized Cd3As2 nanostructures with different morphologies, including triangular and hexagonal nanoplates, nano-octahedrons, and starlike nanostructures. The nanostructures are of high crystalline quality and dominated by {112} exposed facets. Similar to 3D graphene, Cd3 As2 nanoplate in contact with metal electrode was demonstrated to generate photocurrent, which may open new perspectives in optoelectronic applications.

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ASSOCIATED CONTENT

S Supporting Information *

AFM results of the Cd3As2 structures, more SEM images of Cd3As2 triangular nanoplates and nano-octahedrons, the percentage of the nanostructures with different morphologies, SEM images of Cd3As2 products synthesized under another set of conditions, the TGA results of Cd3As2 source and the assynthesized nanostructures, the surface free energy of different crystal planes, and the ideal equilibrium shape of Cd3As2 nucleus. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.cgd.5b00399.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-10-6276-7424. Fax: +86-10-6275-1615. E-mail: [email protected]. Author Contributions #

C.Z.L. and R.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by MOST (Nos. 2013CB934600, 2013CB932602) and NSFC (Nos. 11274014, 11234001, 11404016).

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REFERENCES

(1) Young, S. M.; Zaheer, S.; Teo, J. C.; Kane, C. L.; Mele, E. J.; Rappe, A. M. Phys. Rev. Lett. 2012, 108, 140405. G

DOI: 10.1021/acs.cgd.5b00399 Cryst. Growth Des. XXXX, XXX, XXX−XXX