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Molecular Doping the Topological Dirac Semimetal Na3Bi across the Charge Neutrality Point with F4-TCNQ Mark T. Edmonds,*,†,∥ Jack Hellerstedt,†,∥ Kane M. O’Donnell,‡ Anton Tadich,§ and Michael S. Fuhrer† †
School of Physics and Astronomy and Monash Centre for Atomically Thin Materials, Clayton Victoria 3800, Australia Department of Imaging and Applied Physics, Curtin University, Bentley, Western Australia 6102, Australia § Australian Synchrotron, 700 Blackburn Road, Clayton, Victoria 3168, Australia ‡
ABSTRACT: We perform low-temperature transport and high-resolution photoelectron spectroscopy on 20 nm thin film topological Dirac semimetal Na3Bi grown by molecular beam epitaxy. We demonstrate efficient electron depletion ∼1013 cm−2 of Na3Bi via vacuum deposition of molecular F4TCNQ without degrading the sample mobility. For samples with low as-grown n-type doping (1 × 1012 cm−2), F4-TCNQ doping can achieve charge neutrality and even a net p-type doping. Photoelectron spectroscopy and density functional theory are utilized to investigate the behavior of F4-TCNQ on the Na3Bi surface.
KEYWORDS: Na3Bi, topological Dirac semimetal, thin film, surface transfer doping
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INTRODUCTION Topological Dirac semimetals (TDS), such as Na3Bi and Cd3As2, are a new class of material consisting of two degenerate Weyl points with opposite topological charge occurring at the same momentum protected by crystal symmetry.1−4 As threedimensional analogues to graphene this opens up the possibility to exploit and study the properties of graphene in a 3D material, such as the unusual electronic and transport properties near the Dirac or charge neutrality point. This includes measuring the size of charge puddling,5,6 the role of charged impurity scattering,7 and van der Waals heterostructure engineering.8,9 At present the majority of transport studies on TDS materials so far have been performed on bulk crystals,10,11 which severely limits the possibility of reaching the Dirac point by any means other than bulk doping. Yet, so far no reports in the literature exist on the successful incorporation of dopants into the Na3Bi lattice. Recently, we demonstrated that thin films of Na3Bi can be grown on insulating α-Al2O3(0001) substrates via molecular beam epitaxy (MBE), and that the intrinsic n-type doping can be tuned by over an order of magnitude from ∼4 × 1013 cm−2 to ∼1 × 1012 cm−2 by varying the growth temperature.12 However, even in the lowest doped samples the doping is still in the homogeneous charge transport regime requiring further © 2016 American Chemical Society
electron depletion to reach the neutrality point. The most common method of tuning the carrier density is electrostatic gating, however, the growth of Na3Bi on a sapphire substrate and its reactivity to ambient conditions prevents lowering the doping via electrostatic gating. Surface transfer doping is another potential method to remove the n-type doping. It provides an alternative route to conventional bulk doping, as it does not require impurities to be introduced into the bulk lattice, and has been successfully implemented across a range of other materials, such as graphene,13−16 topological insulators,17−19 and organic−organic heterostructures.20,21 In the case of acceptor doping, high electron affinity molecules deposited onto the surface result in the transfer of electrons from the substrate into the molecular overlayer. Here, we study the effect of molecular doping using the high electron affinity molecule tetrafluorotetracyanoquinodimethane (F4-TCNQ) (electron affinity 5.24 eV) on 20 nm Na3Bi films. Films possessing different levels of intrinsic n-type doping are studied using low-temperature transport measurements to measure the change in carrier density, resistivity and mobility Received: March 18, 2016 Accepted: June 7, 2016 Published: June 16, 2016 16412
DOI: 10.1021/acsami.6b03312 ACS Appl. Mater. Interfaces 2016, 8, 16412−16418
Research Article
ACS Applied Materials & Interfaces
Figure 1. Transport properties of Na3Bi thin films as a function of F4-TCNQ coverage. Carrier density as a function of F4-TCNQ coverage for Na3Bi samples grown at (a) 120−250, (b) 120−345, and (c) 120−360 °C. Sheet resistivity as a function of F4-TCNQ for Na3Bi samples grown at (d) 120−250, (e) 120−345, and (f) 120−360 °C.
occurs at ∼0.2 nm F4-TCNQ coverage, and represents a total electron depletion of 4.44 × 1013 cm−2 (sample 1) and 1.17 × 1013 cm−2 (sample 2) consistent with the Fermi level moving toward the Dirac point. As electrons are depleted the sheet resistivity, ρ□, for both samples in Figure 1d−e increases monotonically from 110 Ω (sample 1) and 200 Ω (sample 2) to saturated values of 1760 Ω and 3300 Ω, respectively, at 0.2 nm. The Hall mobility, μ = (nHeρ□)−1 (not shown) for sample 1 increases from 1234 cm2 V−1 s−1 as-grown to 1723 cm2 V−1 s−1, while the mobility for sample 2 decreases from 2365 cm2 V−1 s−1 to 1400 cm2 V−1 s−1. This monotonic decrease in density and increase in sheet resistivity without significantly degrading the mobility are similar to that observed for the molecular doping of the topological insulator Bi2Se3.17 The decrease in mobility in sample 2 will be discussed later in conjunction with the results from sample 3. In comparison to the monotonic decrease in Hall carrier density and monotonic increase in resistivity for samples 1 and 2, sample 3 shows more complicated behavior. Figures 1c and f plot the density and sheet resistivity respectively for the 120− 360 °C film (sample 3). Figure 1c shows an initial increase in nH from the as-grown value with increasing coverage, followed by an abrupt decrease and change in sign to p-type conduction. With further coverage between 0.1 and 0.2 nm the carrier density then switches back to n-type and remains there for the remaining coverages. The resistivity [Figure 1f] also shows non-monotonic dependence on coverage, first increasing rapidly from 664 Ω (as-grown) to a peak at 7600 Ω (0.025 nm coverage), then decreasing between coverages of 0.05 and 0.1 nm before increasing again to reach 7830 Ω at 0.9 nm F4TCNQ coverage.
as a function of F4-TCNQ coverage. High-resolution photoelectron spectroscopy is then utilized to probe the interaction of F4-TCNQ with the Na3Bi surface.
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RESULTS AND DISCUSSION Transport Measurements. In Figure 1, the responses of the carrier density and the sheet resistivity for three different Na3Bi thin films of thickness t = 20 nm grown on α-Al2O3 to the deposition of F4-TCNQ are measured. Na3Bi films were grown at 120−250 °C (sample 1), 120−345 °C (sample 2), and 120−360 °C (sample 3) (growth details are discussed in the Experimental Methods section). The ex situ prepared corner contacts allow us to make van der Pauw measurements of the sheet resistivity and Hall coefficient in a perpendicular magnetic field up to 1 T. The Hall effect is used to measure the 2D Hall carrier density nH = −e/RH, where RH is the Hall coefficient. Note that throughout this work we discuss 2D carrier density as the most relevant quantity to surface transfer doping which occurs at a two-dimensional interface. The initial Hall carrier densities for samples 1−3 are 4.64 × 1013 cm−2, 1.29 × 1013 cm−2 and 1.75 × 1012 cm−2 respectively. Figure 1a− c plots the evolution of the carrier density for samples 1−3, respectively, and Figure 1d−f plots the corresponding sheet resistivity for each sample with increasing F4-TCNQ coverage. We begin by examining the change in carrier density and resistivity of the samples with high initial doping, sample 1 and sample 2, with increasing F4-TCNQ coverage. For both samples shown in Figure 1a−b, the carrier density decreases monotonically from as-grown densities of 4.6 × 1013 cm−2 (sample 1) and 1.3 × 1013 cm−2 (sample 2) to saturated values of 2.0 × 1012 cm−2 and 1.3 x1012 cm−2, respectively. Saturation 16413
DOI: 10.1021/acsami.6b03312 ACS Appl. Mater. Interfaces 2016, 8, 16412−16418
Research Article
ACS Applied Materials & Interfaces This nonmonotonic evolution of Hall carrier density and resistivity can be explained by considering the homogeneous and inhomogeneous charge regimes of Dirac semimetal materials. In a transport measurement the neutrality point manifests in a pronounced peak in the resistivity and a change in the sign of the Hall effect, both of which are seen in Figures 1c and f. The strange behavior of the Hall carrier density, where it increases after the first F4-TCNQ coverage can also be understood within the same framework. Near the neutrality point the screening of disorder is poor, and the charge carriers are distributed inhomogeneously in the sample, which can be considered as a random distribution of electron and hole puddles.22,23 These competing regions of electrons and holes manifest in a decrease to the Hall coefficient RH and an increase in nH, which overestimates the true carrier density and diverges positively (negatively) as the neutrality point is approached from positive (negative) carrier density. This has been observed on other Dirac semimetal materials with neutrality points such as graphene8,24 and the topological insulator Bi2Se3.18 The neutrality point is when the resistivity and Hall signal are most sensitive to the introduction of impurities, which is the most likely explanation of the transition from p-type back to n-type observed between 0.1 and 0.2 nm. This could be a result of thermally cycling the samples between transport measurement and F4-TCNQ deposition. We note that electron depletion due to surface transfer doping with F4-TCNQ on 20 nm thick Na3Bi thin films may give rise to inhomogeneous doping through the thickness of the sample. This is because electron depletion in the near surface regime has the potential to give rise to significant band bending and a nonuniform distribution of charge. In this case, the Hall effect would be a mobility-weighted average over the depthdependent carrier density. However, as we observe that the mobility is only weakly dependent on doping, the measurement of a p-type Hall signal strongly indicates that the entirety of the film has a net p-type doping, and certainly the surface region is p-type. More work, including transport measurements at larger magnetic fields, and thickness-dependent measurements, could clarify the depth profile of the doping. Photoelectron Spectroscopy. We now turn to highresolution photoelectron spectroscopy to probe the interaction of F4-TCNQ with the Na3Bi surface for a Na3Bi sample grown at 120−340 °C. Figure 2 plots the Bi 5d and Na 2p core levels at a photon energy of 100 eV. Accounting for the higher photoionization cross section of the Bi 5d core level than the Na 2p core level at this photon energy gives the expected 3:1 ratio of Na to Bi in Na3Bi. The Na3Bi hexagonal P63/mmc phase is shown in the inset of Figure 2, where each unit cell is comprised of two stacked Na-(Na/Bi)-Na triple layer structures that have a 60 degree rotation between the adjacent triple-layer. In Figure 3, the response of the Fermi level shift to the deposition of F4-TCNQ is measured using high-resolution surface-sensitive photoelectron spectroscopy of the Bi 5d core level (at photon energy hv = 100 eV). The as-grown Bi 5d core level consists of the characteristic doublet representing the Bi 5d5/2 and 5d3/2 orbitals with peak positions of 23.01 and 26.04 eV, respectively, where the Gaussian width is 0.48 eV and the Lorentzian width is 0.13 eV. There is only a single component within each doublet, consistent with the crystal structure of Na3Bi where every Bi atom is bonded to Na. Upon F4-TCNQ deposition the Bi 5d5/2 and Bi 5d3/2 peak positions shift to lower binding energy (i.e., the Fermi energy moves toward the Dirac point) with increasing coverage and reach saturated
Figure 2. Photoelectron spectroscopy of a 20 nm Na3Bi film on αAl2O3 (0001) grown at 120−340 °C (taken at hv = 100 eV) showing the Bi 5d and Na 2p core levels. The crystal structure of Na3Bi with P63/mmc symmetry is also shown, with the surface terminated Na atom in green, the remaining Na atoms in gold and the Bi atoms in purple.
Figure 3. Molecular coverage-dependent Fermi energy shift measured with photoelectron spectroscopy. Bismuth 5d core level taken at hv = 100 eV at selected F4-TCNQ coverages. Inset: Fermi-level shift plotted as a function of F4-TCNQ coverage.
positions at 0.2 nm coverage, consistent with the transport measurements in Figure 1. This saturation in the Fermi level shift is due to the transfer of charge leading to the formation of a doping-induced interface dipole. This pushes the Fermi level of the Na3Bi above the lowest unoccupied molecular orbital (LUMO) of the F4-TCNQ making it energetically unfavorable for charge transfer, thus causing a saturation in the Fermi level shift and consequently no further change in the carrier density as observed in Figure 1.17,25 This shift is plotted as a function of F4-TCNQ coverage in the inset of Figure 3, with a total shift of 60 ± 30 meV observed. No additional components appear as a result of F4-TCNQ deposition and the attenuation in the Bi 5d intensity due to the F4-TCNQ overlayer, demonstrating that F4-TCNQ does not chemically interact with the Bi in Na3Bi. The shift in Fermi energy is reasonably consistent with that inferred from the shift in doping for Sample 2 grown at the same temperature [Figure 1b]; assuming a Fermi velocity of 16414
DOI: 10.1021/acsami.6b03312 ACS Appl. Mater. Interfaces 2016, 8, 16412−16418
Research Article
ACS Applied Materials & Interfaces 1.4−2.4 × 105 ms−1,3,12 the doping in Figure 1b corresponds to an initial EF of 40−70 meV above the Dirac point and final EF of 20−30 meV above the Dirac point. In Figure 4 the response of the Na 2p core level (at photon energy hv = 100 eV) to F4-TCNQ deposition is measured with
photon energy to increase the mean free path) reveal that this additional Na(2) component is a surface-related feature, as the Na(2) component decreases in intensity when the mean free path is increased. To determine the origin of this additional surface related component, DFT calculations were performed on the Na3Bi hexagonal P63/mmc phase (inset of Figure 2) to simulate the XPS Na 2p core level shifts for the different sodium environments in the unit cell (details in the Experimental Methods). These calculations show that the main component is composed of bulk interlayer Na and Na bonded to Bi in the hexagonal structure, while the additional component is related to surface terminated Na atoms. The DFT calculated peak separation between the two components is 0.66 eV, in excellent agreement with the 0.63 eV measured experimentally. We now turn to the effect of F4-TCNQ deposition on the Na 2p core level at selected coverages of 0.1, 0.3, and 0.6 nm in the lower panels of Figure 4a. The Na(1) component follows the shift to lower binding energy observed in the Bi 5d core level as a result of F4-TCNQ deposition, and saturates at a coverage of 0.2 nm. However, unlike the Bi 5d core level in Figure 3 an additional component labeled Na(3) develops with increasing F4-TCNQ coverage. Na(3) is fitted with the Na 2p3/2 at 32.0 eV and Na 2p1/2 at 32.16 eV, representing a separation of 1.27 eV from the main Na(1) component. With increasing F4-TCNQ coverage the ratio of Na(3) to Na(2) also increases, along with an overall decrease in Na(1). Both components become significantly broadened at higher coverages with the Gaussian width reaching values of 0.7 and 1.3 eV for Na(2) and Na(3) respectively at 0.6 nm F4-TCNQ coverage. The emergence of Na(3) is independent of the charge transfer process, as the saturation in doping occurs at 0.2 nm, whereas the Na(3) component increases significantly between the 0.3 and 0.6 nm coverages. In Figure 4b, we utilize depth-dependent photoelectron spectroscopy of the Na 2p core level at 850 eV to investigate whether Na(3) is a surface-related component. A large increase in the ratio of Na(1) to Na(3) is observed with the increasing mean free path, demonstrating that Na(3) is confined to the surface Na atoms. Furthermore, as no additional peaks arose in the Bi 5d spectra in Figure 3, it is clear that Na(3) is only related to an interaction between the F4-TCNQ molecule and the surface Na atoms. To elucidate the origin of the Na(3) component, density functional theory calculations were performed in order to simulate the optimized geometry of an F4-TCNQ molecule on the surface of Na3Bi. Details of the calculations are found in the Experimental Methods section. Figure 5a shows the c-axis projection of the Na3Bi lattice, where the green atoms represent the surface Na, yellow atoms represent the remaining Na within the unit cell (hexagonal and interlayer Na) and purple atoms represent the Bi. Figure 5b shows the optimized geometry of the Na3Bi with an F4-TCNQ molecule on the surface. It is immediately clear that the surface-terminated Na atoms directly under the F4-TCNQ undergo a large displacement from the original lattice site, with the nearest neighbor surface Na also slightly perturbed from the bare Na3Bi configuration. This displacement would give rise to a nonuniform surface potential and result in increased photoelectron scattering, as observed in the significant broadening of the Na(2) and Na(3) surface components in Figure 4. Furthermore, with increasing F4TCNQ coverage an increasing number of surface Na atoms will be displaced, explaining the increase in the ratio of Na(3) to Na(2), which are both related to surface-terminated Na. To
Figure 4. High-resolution XPS of the Na 2p core level of Na3Bi as a function of F4-TCNQ coverage. (a) Na 2p core level taken at hv = 100 eV where the upper panel corresponds to the as-grown surface followed by sequential coverages of F4-TCNQ at 0.1 nm, 0. 3 and 0.6 nm, respectively. (b) Na 2p core level of Na3Bi with 0.6 nm of F4TCNQ coverage taken at hv = 850 eV.
photoelectron spectroscopy. The upper panel of Figure 4a plots the as-grown Na3Bi film, consisting of a primary component labeled Na(1) and a small secondary component labeled Na(2) to +0.63 eV higher binding energy. Both Na(1) and Na(2) are fit with a doublet to reflect the 0.16 eV spin−orbit splitting of the Na 2p3/2 and Na 2p1/2 levels where the Gaussian and Lorentzian width are 0.36 and 0.13 eV, respectively. Photonenergy dependent spectra not shown (i.e., increasing the 16415
DOI: 10.1021/acsami.6b03312 ACS Appl. Mater. Interfaces 2016, 8, 16412−16418
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Figure 5. (a) Optimized structure of the Na3Bi lattice, and (b) with a physisorbed F4-TCNQ molecule on the surface. See also Figure 2 inset for an oblique view of the crystal structure with the same coloring of the atoms. growth method as reported previously for Na3Bi thin film growth,12 and for Bi2Se3 growth.26,27 High-purity Bi (99.999%) and Na (99.95%) were deposited at rates of ∼0.05 and ∼1 Å/s, respectively. During growth the chamber pressure was less than 5 × 10−9 mbar. Transport Measurements. After growth, samples for transport measurements were transferred in UHV from the growth chamber to the analysis chamber. Transport characterization was performed using a van der Pauw geometry and standard DC electrical measurements in a magnetic field up to 0.9 T at 5 K. After each measurement the sample was returned to the growth chamber and F4-TCNQ was sequentially deposited at a rate of ∼0.03 Å/s (determined using a quartz crystal microbalance) with the Na3Bi film remaining at 30 °C, before subsequent re-characterization in the analysis chamber. Photoelectron Spectroscopy. Samples for photoelectron spectroscopy were grown in the preparation chamber of the Soft X-ray Beamline at the Australian Synchrotron using the same growth conditions as outlined above, and subsequently transferred under UHV to the analysis chamber for measurement. After each measurement the sample was returned to the preparation chamber for sequential F4-TCNQ deposition. F4-TCNQ coverage was determined using a quartz crystal microbalance and then confirmed from the attenuation of the Bi 5d core level. The size of the illuminated area is approximately 100 μm × 20 μm; multiple spots on each thin film sample were measured. The peak intensity of the Bi and Na core levels were found to be uniform across the sample. Both the Bi 5d and Na 2p core level components of Na3Bi at a photon energy of 100 eV were measured to ensure high surface sensitivity with an overall measurement uncertainty of ±30.0 meV, and at higher photon energies to characterize the depth dependence. The binding energy scale of all Na3Bi related spectra are referenced to the Fermi energy (EF), determined using either the Fermi edge or Au 4f core level of an Au reference in electrical contact with the sample. Core level spectra were analyzed using a Shirley background subtraction,28 and then peak fitted using Voigt functions for each peak component. Density Functional Theory. Slab calculations were carried out using the FHI-aims density functional theory code29 on a 128-atom 4 × 4 periodic slab consisting of six atomic layers and a 21 Å vacuum gap. The default tight numerical local orbital basis set was used with the AM05 exchange-correlation functional,30 collinear spin and scalar relativistic approximations. Relaxations were carried out to obtain residual forces below 0.01 eV/Å on a 6 × 6 × 1 k-point grid. For relaxation the lowest layer of sodium atoms were fixed at bulk positions to simulate an infinitely thick slab, all other atoms were free to move. Density of states calculations were carried out on a 36 × 36 × 1 k-point grid with an energy broadening of 0.05 eV. Bulk calculations were carried out on the Na3Bi unit cell using the same settings as for the slab but with a k-point grid of 24 × 24 × 1 to reflect the smaller cell size. To simulate XPS spectra, total energy calculations were carried out using the Projector Augmented Wave (PAW) method within the Quantum Espresso code.31 A plane wave cutoff of 680 eV was used along with a 6 × 6 × 1 k-point grid as above. For each surface Na site in turn a SCF calculation was carried out with the occupancy of the specific Na 2p orbital restricted to a single electron to simulate X-ray induced ionization of that site using a 2p core-hole pseudopotential.
confirm that Na(3) is a result of this displacement, DFT calculations were performed on the structure in Figure 5(b) to simulate the XPS Na 2p core level shifts for the different sodium environments. The DFT calculated peak separation between the bulk component and the surface Na atoms is 0.71 eV, which is +50 meV higher in binding energy than the surface Na atoms for the Na3Bi lattice. This indicates that the distortion of the surface Na as a result of F4-TCNQ physisorption onto the Na3Bi surface does lead to a charge localization and a shift to higher binding energy. This shift is significantly less than the 1.27 eV measured experimentally. However, it should be noted that the simulation reflects an isolated F4-TCNQ molecule and is not necessarily indicative of the collective surface Na atom distortion as the F4-TCNQ coverage approaches a monolayer in thickness which corresponds to 0.4 nm.
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CONCLUSION In conclusion, we identify the molecule F4-TCNQ as a highly effective acceptor dopant for the topological Dirac semimetal Na3Bi. Transport measurements performed at low-temperature (5K) demonstrate that in excess of 1013 cm−2 electrons can be depleted from the Na3Bi. For Na3Bi samples that possess a low intrinsic doping this electron depletion is sufficient to reach the charge neutrality point, and even cause a net p-type doping. High-resolution synchrotron-based photoelectron spectroscopy measures a 60 meV shift of the Fermi level to lower binding energy, consistent with the electron depletion observed in transport measurements. The deposition of F4-TCNQ onto the Na3Bi surface also results in a displacement of the surface Na atoms from the expected lattice site; this interaction is independent of acceptor doping and does not cause a significant degradation in mobility. This confirms that F4TCNQ is only physisorbed onto the Na3Bi surface, and offers a nondestructive doping mechanism. These findings open up new pathways to study topological Dirac semimetals at the charge neutrality point without the need to find bulk acceptor atoms that can be successfully incorporated into the Na3Bi lattice.
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EXPERIMENTAL METHODS
Sample Growth. Na3Bi films were grown via molecular beam epitaxy (MBE) on sapphire (α-Al2O3) samples (Shinkosha Japan) that had been annealed in atmosphere at 1350 °C and then pure oxygen atmosphere at 1050 °C in order to achieve an atomically flat surface. Ti/Au (5/50 nm) contacts were deposited on the corners of the substrate, and wirebonded to a contact busbar on the sample plate. Samples were then introduced into ultrahigh vacuum (UHV) immediately after wire bonding to minimize exposure to ambient conditions, and then annealed to 400 °C for 1 h to remove adsorbed atmospheric species. Na3Bi films were then grown using a two-step 16416
DOI: 10.1021/acsami.6b03312 ACS Appl. Mater. Interfaces 2016, 8, 16412−16418
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The difference in all-electron total energy between the ground and ionized calculations reflects the ionization energy of the state.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ∥
These authors contributed equally to this work
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.T.E, J.H., and M.S.F. are supported by M.S.F.’s ARC Laureate Fellowship (FL120100038). M.T.E is also supported by his ARC DECRA fellowship (DE160101157). Photoelectron spectroscopy measurements were performed at the soft X-ray beamline of the Australian Synchrotron. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). This work was supported by the Multimodal Australian ScienceS Imaging and Visualisation Environment (MASSIVE) (www.massive.org. au).
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REFERENCES
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