Tunable Lattice Constant and Band Gap of Single - ACS Publications

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Tunable Lattice Constant and Band Gap of Single- and Few-Layer ZnO Junseok Lee, Dan C. Sorescu, and Xingyi Deng J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00432 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 24, 2016

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The Journal of Physical Chemistry Letters

Tunable Lattice Constant and Band Gap of Singleand Few-Layer ZnO

Junseok Lee,*,†,‡ Dan C. Sorescu,† and Xingyi Deng†,‡ †

National Energy Technology Laboratory, U. S. Department of Energy, Pittsburgh, PA 15236,

United States ‡

AECOM, P.O. Box 618, South Park, PA 15129, United States

*Corresponding author: [email protected]

Abstract Single and few-layer ZnO(0001) (ZnO(nL), n = 1 - 4) grown on Au(111) have been characterized via scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. We find that the in-plane lattice constants of the ZnO(nL, n ≤ 3) are expanded compared to that of the bulk wurtzite ZnO(0001). The lattice constant reaches a maximum expansion of 3% in the ZnO(2L) and decreases to the bulk wurtzite ZnO value in the ZnO(4L). The band gap decreases monotonically with increasing number of ZnO layers from 4.48 eV (ZnO(1L)) to 3.42 eV (ZnO(4L)). These results suggest that a transition from a planar to the bulk-like ZnO structure occurs around the thickness of ZnO(4L). The work also demonstrates that the lattice constant and the band gap in ultrathin ZnO can be tuned by controlling the number of layers, providing a basis for further investigation of this material.

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The advent of graphene followed by the discovery of its unique properties1, 2 has initiated the exploration of other two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs).3, 4 The field of 2D materials has become one of the focal points in material science, leading to ample opportunities for creating novel nanostructures with tailored electronic and catalytic properties. In addition to TMDs, other classes of materials such as layered metal oxides can also be classified as 2D materials.5,

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The diverse applicability of metal oxides in areas

ranging from catalysis to electronics makes them appealing candidates for investigation.7 Among metal oxides, ZnO is one of the most studied materials in its bulk/powder form due to its applications in semiconductor devices8 and its importance in methanol synthesis from syngas mixtures.9 ZnO is a wide band gap semiconductor (Eg = 3.37 eV) with a large exciton binding energy.10 Unlike TMDs, free-standing layers of ultrathin ZnO were difficult to obtain, but recent studies showed that crystalline ZnO nanosheets11 or graphene-like ZnO membranes12 can be prepared, opening up new opportunities for further research. At ambient conditions, the most stable form of ZnO is the wurtzite structure (a schematic is shown in Figure 1a). The polar wurtzite ZnO(0001)-Zn and ZnO(0001)-O surfaces are unstable, 2


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causing reconstructions that stabilize these surfaces.13, 14 However, in the ultrathin ZnO regime, a different stabilization mechanism is at work; where depolarization takes place via the formation of the unreconstructed planar sheets (Figure 1b).15, 16 Structurally, the bonding configuration of the Zn and O atoms in the ZnO layers changes from sp3 tetrahedral in the wurtzite structure to sp2 trigonal coordination in the planar structure. For free-standing ZnO, the critical thickness up to which the planar structure is stable was predicted computationally to be ~8 layers,15, 17 while in the case of ZnO layers supported on a Ag(111) surface, the planar structure was calculated to be stable up to a bilayer.18 Experimentally, it was found that the ZnO layers on Ag(111) and Pd(111) adopt bulk-like structures at a thickness of 4 layers.16, 19 A material’s band gap is another important property which can impact its application in photovoltaics, sensing, and photocatalysis. The band gap of bulk ZnO can be modified via alloying, doping, and defect engineering.8, 20, 21 For ultrathin ZnO, it was calculated that one could tune the band gap via strain engineering in the ZnO/substrate system,17 similar to the results obtained for the monolayer TMDs.22, 23 For stacked layers of ZnO, the band gap was calculated to decrease as the number of layers increased, eventually approaching the bulk value.18, 24, 25 Experimentally, for thicker ZnO films (> 15 ML) grown on Au(111), the band gap was measured to be 2.8 eV.26,

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However, a systematic measurement of the band gap

modification of ZnO as a function of the number of ZnO layers in the few-layer regime is still lacking in the literature. In this work, we report on the study of the structural and electronic properties of few-layer ZnO systems. (Throughout this work, we refer to ZnO(nL) as the n-layer thick ZnO structure. For example, ZnO(1L) represents the ZnO monolayer.) We have found that the in-plane lattice constants of the ZnO(nL, n ≤ 3) are all expanded with the ZnO(2L) having the largest expansion

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(3%). We have also found that the band gap of ultrathin ZnO decreases monotonically as the thickness of ZnO increases, reaching a value close to the bulk band gap at ZnO(4L). Briefly, STM/STS experiments were performed in a low-temperature STM system (Omicron) at liquid nitrogen temperature (T = 77.4 K) using several tungsten tips. The bias voltage refers to the sample voltage. To obtain reliable physical dimensions in the STM measurements, calibration of the piezo scanner was carefully performed prior to the measurements of the lattice constants and of the Moiré pattern periodicities (Figure S1). Reactive deposition of Zn in NO2 was employed to grow the ZnO layers as shown in Figures S2 and S3. (See SI for experimental and theoretical details.)

Figure. 1. Schematics of the wurtzite (a) and the planar (b) ZnO structures. (c) STM image of a ZnO(1L) structure on Au(111) (+1.0 V, 50 pA, 100 × 100 nm2). The yellow arrows indicate the close packed direction of Au(111) substrate. Two red ellipses indicate the areas of disturbed

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Moiré patterns. The inset shows the apparent height (~2 Å) along the white solid line. At the lower right side of the image, a part of a ZnO(2L) structure is also visible. (d) High resolution STM image (+5 mV, 1.5 nA, 10 × 10 nm2) of the area indicated in the square in (c). Atomically resolved ZnO lattice and the Moiré pattern are clearly visible. The unit cells of ZnO and Moiré pattern are marked in red and green, respectively. (e) A 2D Fourier transform of the STM image in (d) with outer spots for ZnO lattice and inner spots for the Moiré pattern near the center. A straight line shows the alignment of the ZnO lattice and the Moiré pattern. The lattice constant and the Moiré periodicity of the ZnO(1L) from the Fourier transform are found to be 3.29 ± 0.02 Å and 24.02 ± 0.43 Å, respectively.

Figure 1c shows an STM image of a triangular ZnO(1L) on the Au(111) substrate. Previously, small isolated ZnO monolayer structures with apparent heights of 2.0 Å have been observed on Pd(111) and Pt(111), but not on Au(111) or Ag(111).16,

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The sides of the triangular

ZnO(1L) are aligned along the close-packed direction of the Au(111) substrate as shown in Figure 1c. The apparent height of the ZnO(1L) (~2 Å) agrees well with that reported in previous studies,16, 19, 28 and suggests that the thickness of the ZnO(1L) is considerably reduced compared to the single step height of the bulk ZnO(0001) single crystal (2.6 Å).30 In our previous work,29 we assigned the feature with an apparent height of 3.5 Å to ZnO(1L) but we revise it to be ZnO(2L) based on the new measurements in the current work. We note, however, that the height measured in STM represents an apparent height at a particular bias voltage. Thus, direct comparison with a real physical dimension should be made with caution. A closer inspection of Figure 1c (red ellipses) reveals that the Moiré patterns in some areas are disturbed due to the unlifted herringbone reconstruction (Figure S4).31

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A high resolution STM image of the ZnO(1L) surface and its 2D Fourier transform image are shown in Figures 1d and 1e, respectively. The STM image shows that the surface of the ZnO(1L) is almost free of defects and/or adsorbates compared to the surfaces of ZnO(0001) or Zn(0001) bulk crystals.14, 30 From the Fourier transform analysis, the lattice constant of the ZnO(1L) is calculated to be 3.29 ± 0.02 Å which is about 1.2% larger than that of the bulk ZnO(0001) lattice constant (3.25 Å). A straight red line drawn through two spots in Figure 1e shows that the ZnO and Au(111) lattices are aligned along the same direction (see Figure S5). Figure 2a shows an STM image of a ZnO structure on Au(111) where up to four layers of ZnO are visible. The height profile along the green solid line is shown in Figure 2b. The average apparent heights of ZnO(nL) (n = 1 - 4) are 2.0 ± 0.1, 3.7 ± 0.1, 5.5 ± 0.1, and 8.0 ± 0.3 Å, respectively (Figure 2c). As the number of ZnO layers increases, the apparent height difference reaches a minimum at the ZnO(2L) (right axis in Figure 2c) suggesting that the top layer of the ZnO(2L) is the thinnest ZnO layer among the four. The apparent height difference between the ZnO(3L) and the ZnO(4L) is almost identical to the single step height of the bulk ZnO structure (dotted line in Figure 2c). This trend resembles the surface x-ray diffraction results found in the ZnO/Ag(111) system.16

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Figure 2. (a) STM image (+1.0 V, 50 pA, 95 × 95 nm2) of a ZnO structure grown on Au(111) using reactive deposition method at T = 465 K. (For larger ZnO structure images, see STM images in SI.) Up to 4 layers of ZnO are visible in the image. The layer numbers are marked on each corresponding layer. (b) Height profile along the solid green line in (a). The apparent height of the average Au(111) surface plane is set to zero. The apparent height measurements have also been performed at V = -1.0 V but produced similar results. (c) The average apparent height (filled circle, left axis) and the height difference (filled square, right axis) between successive ZnO layers. The dotted line indicates the single step height of the bulk ZnO crystal. The apparent height of the ZnO(1L) itself is used to indicate the height difference between the ZnO(1L) and Au(111) substrate. (d) The lattice constant of ZnO (filled circle, left axis) and the Moiré periodicity (filled square, right axis) as a function of the number of ZnO layers. The axis for the percent lattice expansion with respect to the bulk ZnO lattice (3.25 Å) is also shown in the upper

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left part. The Au lattice constant (filled diamond) is derived from ( ) for the

case of 0° rotation angle between the Au substrate and the ZnO overlayer (where dAu(111), dZnO, 7



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and dMoire are the lattice constants of Au(111), ZnO, and Moiré pattern, respectively).32 The dotted blue line indicates the lattice constant of the wurtzite ZnO bulk and the dotted red line indicates the lattice constant of the ideal Au(111) surface. (e) A schematic of an optimized (9 × 9) ZnO(1L) overlayer on a (16 × 16) three-layer Au slab from DFT calculations. A similar Au supercell model has been used for the ZnO(nL) (n = 2 - 4) structures. The lattice constant of each ZnO(nL) layer was obtained by averaging all the nearest neighbor O-O distances in the top layer excluding the edge area. (f) Comparison of the experimental (black filled squares) and DFT calculated (red filled circles) lattice constants as a function of the number of ZnO layers. The axis on the right indicates the percentage lattice expansion with respect to the bulk ZnO lattice constant.

Figure 2d shows the average in-plane lattice constants of ZnO(nL) (n = 1 – 4) and their Moiré pattern periodicities. The lattice constants of ZnO and the Moiré periodicity of each ZnO layer were obtained by employing a 2D Fourier transform of the corresponding high resolution STM image as demonstrated in Figures 1d and 1e. Interestingly, the lattice constant as a function of the number of the ZnO layers shows a maximum at the ZnO(2L), and then decreases to almost the bulk ZnO value at the ZnO(4L). This suggests that there is an inverse relationship between the in-plane lattice constant and the thickness of the ZnO layer. A similar inverse relationship is found in Figure 2d between the measured periodicity of the Moiré pattern and the lattice constant. This finding is expected from the mathematical relationship between the periodicities of the substrate and the overlayer assuming that the lattice constant of the Au(111) substrate does not change.32 Indeed, the calculated lattice constant of Au(111) underneath each ZnO layer is close to the ideal unreconstructed lattice constant (2.88 Å) of the Au(111) surface. According to the

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results in Figures 2c and 2d, the ZnO(2L) is the thinnest among the four layers and has the most expanded in-plane lattice constant compared to the bulk ZnO (~3% expansion). While a previous study suggests a fixed lattice expansion ratio regardless of the number of the ZnO layers,16 our layer-specific measurements based on STM clearly demonstrate that the lateral lattice expansion occurs at varying degrees in these ZnO layers, reflecting changes in the local energy relaxation landscape. In order to model the lateral variations in the lattice constants of the ZnO structures, a slab model containing a (9 × 9) ZnO overlayer on a larger (16 × 16) Au slab was considered. Previous DFT studies of ZnO supported on metallic substrates have employed a so called ‘coincidence’ structure to describe the Moiré patterns and to simplify the modelling process via periodic boundary conditions. For example, a 7/8 coincidence structure between ZnO and the substrate was employed.29 Once a coincidence ratio is selected,18, 29 the lateral lattice constant is forced to be fixed at a certain ratio to the substrate lattice constant due to the periodic boundary condition. Thus, the change in the lateral lattice constant measured in the experiment cannot be reproduced in such periodic models. The (9 × 9) ZnO / (16 × 16) Au slab model in Figure 2e allows relaxation of the ZnO lattice in both lateral and vertical directions. The optimizations have been performed using DFT calculations with a mixed Gaussian and plane-wave formalism as implemented in the Quickstep module of the CP2K program.33, 34 The PBE functional35 corrected to include Grimme-D3 long range dispersion corrections was used in the calculations.36 The calculated lattice constants of ZnO layers are shown in Figure 2f. It should be noted that upon relaxation of the (9 × 9) ZnO / (16 × 16) Au structure, there are some distortions near the edges of ZnO structures but the overall ZnO structure remains planar. In the case of ZnO(4L), a very small corrugation exists with a preference for O-termination. For comparison, the experimental

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results are also plotted in the same figure. The computational results predict that the maximum lattice expansion to take place in the ZnO(3L), whereas the experimental measurements have shown that the ZnO(2L) has the largest lattice constant (~3% expansion with respect to the bulk value). The origin of the discrepancy is not clear but it might arise either from the significantly smaller size of the computational model compared to the real ZnO structure which encompasses multiple Moiré regions, or from other modifications that were not considered in the current computational model such as oxygen vacancies37 and/or the presence of adsorbates such as hydroxyl species. Nevertheless, both experimental and theoretical results suggest that the lateral lattice expansion from the bulk value diminishes with the increasing number of ZnO layers after reaching a maximum. We believe this is due to a delicate balance in the structural stabilization mechanism between the depolarization by interlayer relaxation versus the formation of thermodynamically more stable bulk structures at thicker layers.

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Figure 3. (a) (Lower panel) Differential conductance (dI/dV) spectrum of the clean Au(111) surface. The spectrum is shown in logarithmic scale. The surface state peak at V = -0.4 eV is marked with an arrow. (Upper panel) Differential conductance (dI/dV) measurements for each ZnO layer (Lock-in amplifier parameters: Vmod = 20 mV, f = 361 Hz, Vset = 1.0 V, Iset = 50 pA). To better determine the band edges, the logarithm of dI/dV signal is taken.38 The positions of the band edges of each spectrum are indicated with dotted lines. The band gap for ZnO(4L), Eg(4L), is also indicated. The measurements have been performed on the surface of ZnO layers, away from the edges and adsorbate features. (b) Comparison of the experimental (black circle) and DFT calculated band gaps as a function of the number of ZnO layers. Calculated results were obtained at PBE-D3, PBE-D3 + U (U = 8.5 eV), and HSE06 (with 0.25 Hartree-Fock exchange) theoretical levels using the average lattice constants obtained from (9 × 9) ZnO / (16 × 16) Au slab model. The dotted line indicates the band gap of the bulk ZnO. (c) Influence of different lattice constants on the band gap. The band gaps were calculated using PBE-D3 functional for three different sets of in-plane lattice constants corresponding to the optimized bulk ZnO structure, the 7/8 ZnO/Au coincidence structure, and the (9 × 9) ZnO / (16 × 16) Au model used in (b), respectively. (d) Projected density of states (PDOS) of ZnO layers on the Au(111) substrate for 7/8 ZnO/Au coincidence structure. These results were obtained using the PBE-D3 + U (U = 8.5 eV) functional. The onset of the conduction band minimum of each ZnO layer is indicated by a vertical black line. The PDOS curves are displaced vertically to enhance the visibility.

The electronic structures of the ZnO layers have been studied using STS. The differential conductance (dI/dV) spectroscopy in the lower panel in Figure 3a shows the dI/dV spectrum of

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the clean Au(111) area, where the surface state peak is clearly visible near V = -0.4 eV.39 The upper panel in Figure 3a shows dI/dV spectra measured at the surface of each ZnO layer. The spectra were taken at positions away from the edges on relatively large ZnO areas (> 10 × 10 nm2) to avoid the effect of the edge states. We took an average of spectra for each ZnO layer, and determined the onset of the valence band maximum (VBM) and conduction band minimum (CBM) by taking the logarithm of the spectra following previous STS work.38 The band gap of the ZnO layers decreases as the number of layers increases, becoming quite similar to that of the bulk ZnO at ZnO(4L) (Figure 3b).40 The CBM decreases by as much as 1 eV as the number of ZnO layers increases while the VBM remains near V = -2 eV with respect to the Fermi level (EF, V = 0). In the dI/dV spectra of ZnO(1L), there is also a feature at V = ~1.75 eV with an unknown nature at this point. The decreasing trend of the ZnO band gap as a function of the number of layers suggests potential tunability of the band gap of ultrathin ZnO layers. Similar results were found for other TMD systems.3, 41, 42 The variation of the band structure as a function of the number of ZnO layers has also been analyzed using DFT calculations. The band gap calculations were performed using plane-wave basis sets generated using the projector augmented wave (PAW) method of Blöch as implemented in VASP code.43 Firstly, we analyzed the variation of the band gap using PBE-D3, PBE-D3+U (U = 8.5 eV), and HSE06 hybrid functionals. The individual layers were fully optimized while the lateral size of the simulation box was fixed at the average lattice dimension obtained from the calculations of the (9 × 9) ZnO / (16 × 16) Au model. For all ZnO layers, the VBM and the CBM are located at the Γ point in the band structure, indicating direct band gaps (see Figure S6). Figure 3b shows the calculated band gaps along with the experimental results. Among different methods considered, the best agreement with the experimental data was

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obtained by hybrid functional HSE06. However, all three functionals predict monotonic decrease of the band gaps as the number of ZnO layers increases. We also analyzed the influence of the in-plane lattice constant change on the band gap of ZnO. Figure 3c shows that there is little change in the predicted band gaps for ZnO(1L) with different lattice constants. Compared to the (9 × 9) ZnO / (16 × 16) Au model, the calculated band gaps are 1.6% and 2.9% lower for the ZnO(4L) with 7/8 coincidence and bulk-like structures, respectively. Thus, the band gap change observed in this work is mainly caused by the stacking of the ZnO layers. Lastly, we considered the effect of the Au substrate on the band gap variation. Relatively low adhesion energies of ZnO(nL, n = 1 - 4) on Au were found in the range of 0.40 – 0.50 eV / ZnO. In addition, a small charge transfer ranging from 0.01 electrons / ZnO (for ZnO(4L)) to 0.02 electrons / ZnO (for ZnO(1L)) was found in the Bader charge analysis. The majority of the transferred charge is found to reside in the first ZnO layer regardless of the thickness of ZnO layer. Overall, both the adhesion energy and Bader charge analyses indicate that the interaction between the ZnO and the Au substrate is weak. We have also analyzed the variation of the projected density of states (PDOS) as shown in Figure 3d. Similar to the trends found for the isolated ZnO layers (Figure 3b), we observe the band gap narrowing as the number of ZnO layers increases.

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Figure 4. (a) STM image of a ZnO structure on Au(111) (82 × 82 nm2, 1.5 V, 50 pA). Up to 4 layers of ZnO are visible. (b) Normalized dz/dV spectra of ZnO layers. The dz/dV spectrum for ZnO(4L) contains noise due to the changes in the tunneling junction. (c)-(f) Differential conductance (dI/dV) maps of the ZnO structure in (a). The base of the original topographic image in (a) is indicated by a dotted triangle in each figure. (c) ZnO(1L), V = 3.2 V. (d) ZnO(2L), V = 2.5 V. (e) ZnO(3L), V = 2.0 V. (f) ZnO(4L), V = 1.6 V.

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The layer-specific electronic properties of ultrathin ZnO layers shown above provide a unique opportunity to distinguish among these nanostructures which would otherwise be difficult to be identified solely based on the apparent height measurements. In Figure 4a, we show the differential conductance maps of a ZnO nanostructure at bias voltages that have been identified in the dz/dV spectroscopy measurements.44, 45 In Figure 4b, the normalized dz/dV spectra are shown for each ZnO layer. The peak positions in Figure 4b correspond to the maximum change in the slope of the z(V) spectra (i.e. the energy where the density of states change is maximum). The dz/dV peak position decreases as the number of the ZnO layers increases, following the same trend as the band gap change in Figure 3a. The differential conductance (dI/dV) maps of the ZnO structure are shown in Figures 4c-4f at the peak energy positions identified in dz/dV spectra. For example, only the ZnO(1L) is revealed at V = 3.2 V in the dI/dV map in Figure 4c. Note that there is almost no intensity in the center of the ZnO structure in Figure 4c because the rate of change in the density of states is minimal at this position for ZnO(2L) – ZnO(4L) as seen by the flat spectra at 3.2 eV in Figure 4b. The ZnO(2L) and ZnO(3L) are also clearly revealed in Figures 4d and 4e, respectively. However, in the dI/dV map of the ZnO(4L) shown in Figure 4f, there is a slight contribution from the ZnO(3L). This is caused by the proximity of the energy positions of the dz/dV peaks of the ZnO(3L) and ZnO(4L) as shown in Figure 4b, where the dz/dV peak for the ZnO(3L) extends into the V = 1.6 eV range. Thus, by employing the dI/dV mapping technique, clear identification of each ZnO layer in a complex multilayer ZnO structure is possible, which could be extended to other nanostructure studies. In summary, we find that the in-plane lattice constant of the ultrathin ZnO is expanded in ZnO(nL, n ≤ 3) compared to the lattice constant of the bulk wurtzite structure, and nearly reaches

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the bulk value in ZnO(4L). The expansion of the lattice reaches a maximum of about 3% at ZnO(2L). The band gap of the ZnO is found to decrease monotonically from 4.48 eV for ZnO(1L) to 3.42 eV for ZnO(4L). These findings are supported by the DFT calculations. The results demonstrate that the lattice constant and the band gap of ultrathin ZnO can be tuned by controlling the number of the layers in the film. Our study provides new insight into the property changes in the few-layer regime of widely used ZnO and establishes a basis for further investigation of this material.

Acknowledgment This technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research under the RES contract DE-FE0004000. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Supporting Information Available: Experimental, theoretical methods, and supporting figures.

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Tunable Lattice Constant and Band Gap of Single - ACS Publications

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