
==== Front
ACS Nano
ACS Nano
nn
ancac3
ACS Nano
1936-0851
1936-086X
American Chemical Society

39178330
10.1021/acsnano.4c02784
Article
Surface Doping and Dual Nature of the Band Gap in Excitonic Insulator Ta2NiSe5
Lee Siwon †‡
https://orcid.org/0000-0002-5116-9987
Jin Kyung-Hwan †§
Jung Hyunjin †‡
Fukutani Keisuke †
https://orcid.org/0000-0002-6075-5101
Lee Jinwon †‡∥
Kwon Chang Il †‡
Kim Jun Sung †‡
https://orcid.org/0000-0001-8499-9488
Kim Jaeyoung †
https://orcid.org/0000-0002-8538-8993
Yeom Han Woong *†‡
† Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), Pohang 37673, Republic of Korea
‡ Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
§ Department of Physics and Research Institute of Physics and Chemistry, Jeonbuk National University, Jeonju 54896, Republic of Korea
∥ Department of Quantum Nanoscience, Kavli Institute of Nanoscience, Delft University of Technology, Delft 2628 CJ, The Netherlands
* Email: yeom@postech.ac.kr.
23 08 2024
10 09 2024
18 36 2478424791
28 02 2024
15 08 2024
15 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Excitons in semiconductors and molecules are widely utilized in photovoltaics and optoelectronics, and high-temperature coherent quantum states of excitons can be realized in artificial electron–hole bilayers and an exotic material of an excitonic insulator (EI). Here, we investigate the band gap evolution of a putative high-temperature EI Ta2NiSe5 upon surface doing by alkali adsorbates with angle-resolved photoemission and density functional theory (DFT) calculations. The conduction band of Ta2NiSe5 is filled by the charge transfer from alkali adsorbates, and the band gap decreases drastically upon the increase of metallic electron density. Our DFT calculation, however, reveals that there exist both structural and excitonic contributions to the band gap tuned. While electron doping reduces the band gap substantially, it alone is not enough to close the band gap. In contrast, the structural distortion induced by the alkali adsorbate plays a critical role in the gap closure. This work indicates a combined electronic and structural nature for the EI phase of the present system and the complexity of surface doping beyond charge transfer.

ARPES
DFT
excitonic insulator
van der Waals material
metal insulator transition
structural transition
Institute for Basic Science 10.13039/501100010446 IBS-R014-D1 Institute for Basic Science 10.13039/501100010446 IBS-R014-Y1 document-id-old-9nn4c02784
document-id-new-14nn4c02784
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pmcExcitons are bosonic quasiparticles in which an electron and a hole are bound by the Coulomb potential. Excitons are observed in a wide range of semiconductors and molecules as excited states with rather short lifetimes but with paramount roles in various photovoltaic and optoelectronic devices,1−8 and photosynthesis.9,10 Moreover, the coherent quantum condensation of excitons has been demonstrated within coupled semiconductor quantum wells11−15 and graphene,16−19 which attracted huge scientific interest. While the weak exciton binding energy in these systems limited the condensation temperature below 1 K, artificial electron–hole double layers were developed to confine interlayer excitons, which have substantially increased exciton binding energy and extremely high condensation temperature up to 100 K.20−22 High-temperature exciton condensates enable scientific and technological possibilities such as excitonic high-temperature superconductors, dissipationless delivery of information, and coherent optoelectronics. An even higher-temperature exciton condensate may be realized in an exotic quantum material of an excitonic insulator (EI), which is a small gap semiconductor or a semimetal with a huge and poorly screened excitonic interaction.23−27 In this class of materials, the exciton binding energy exceeds its band gap and the exciton condensate becomes the ground state, which is sustainable even at room temperature.26,28

Throughout the quest for higher-temperature condensation and the exploitation of excitons for various applications, control over the excitonic interaction, reflected by the exciton binding energy, is crucial. However, the exciton binding energy is largely given by fundamental material properties, such as the band gap and the dielectric constant, so systematic tuning over it has been challenging for a given material. For example, the exciton binding energy in bulk semiconductors could only be marginally tuned by lattice strain.29 For exciton condensates in EIs, an early work reported only a transient and marginal perturbation over the exciton binding energy using laser pulses.30 This is in contrast to the recent integration of a 2D monolayer semiconductor into a field-effect transistor device, which has made possible the injection of huge amounts of charges to induce substantial tuning of exciton binding energy.31,32 On the other hand, the strong electron doping by surface adsorbates was recently reported to tune substantially the excitonic energy gap.33,34 However, the gap-tuning mechanism has not been clear with the possibility of strong phononic interaction.35 This issue, that is, how the excitonic gap is controlled by electron doping, is deeply related to the uncertainty of the electronic transition (gap formation) mechanism itself in an excitonic material,36 which definitely requests further studies.

So far, mainly two materials, TiSe2 and more recently Ta2NiSe5 (TNS), have been actively discussed as strong candidates for an EI. In terms of material properties, TNS seems more attractive than a highly debated case of TiSe224,25,37 since the former is a direct gap material and its transition temperature is much higher (see Figure S1 in the Supporting Information for a detailed comparison). TNS is a van der Waals layered material with one-dimensional atomic chain structures of Ta and Ni within each layer (Figure 1a). TNS has an orthorhombic bulk structure above Tc ∼ 328 K, where a transition occurs to a monoclinic structure38 with an insulating band gap measured optically as 160 meV.26 Breaking the mirror symmetry through the structural transition allows the hybridization of conduction (Ta 5d) and valence (Ni 3d) bands (Figure 1c), which is required for the exciton formation.39 The valence band dispersion below Tc is shown in Figure 1b as measured by angle-resolved photoelectron spectroscopy (ARPES) in two different linear polarizations of the incident light. ARPES spectra exhibit strong polarization dependence due to mirror symmetry and selection rules. The experimental setup and overall polarization-dependent ARPES spectra are illustrated in Figure S2 (Supporting Information). Since the horizontal LH (vertical LV) polarization has only the xz(y) component in the first Brillouin zone (BZ), the conduction (valence) band coming mainly from the Ta 5dy (Ni 3dx) orbital becomes invisible as reported before.40,41 The previous data were acquired using nonpolarized light or photons with both y and z components, which mix valence- and conduction-bands contributions.33,35 Due to the interband hybridization, the valence band top has a strong contribution from the conduction band of Ta 5d, which leads to the vanishing (enhanced) ARPES intensity in the LH (LV) polarization. The density functional theory (DFT) calculation for the monoclinic structure (Figure 1c) reproduces the inverted (hybridized) orbital character of the valence band top while it underestimates the band gap size substantially (only 60 meV out of the optical gap of 160 meV26 or the tunneling gap of 300 meV41). This is well expected since the excitonic many-body interaction cannot be sufficiently included in DFT.42−45 Therefore, the band gap in the present calculation is thought to be due mainly to the structural degree of freedom. The small gap opening by the structural transition from the orthorhombic to the monoclinic structure was already discussed in the previous calculations.46−48

Figure 1 Atomic and electronic structures of the TNS. (a) Atomic structure of K-adsorbed TNS [top (side) view at the bottom (top) panel]. 1D atomic chains of Ta and Ni run along a direction and each layer is stacked through van der Waals interaction along b direction. K favors sitting on top of one of the Ni chains. (b) ARPES spectral intensity maps of pristine TNS along the a direction in three different polarizations. The valence band (valence band top) is visible for linear-horizontal (LH)-pol (linear-vertical (LV)-pol-pol) in the first Billouin zone (BZ) but both bands are visible in the third BZ, where the strict polarization selection does not work. (c) Calculated bulk band structure of monoclinic TNS along the a direction. The states projected onto Ta 5d (Ni 3d) orbitals are depicted in red (orange) color, which explains the polarization dependence shown in (b). (d) Schematic interpretations for the evolution of the band dispersion and the band gap (2Δ) upon alkali metal doping. Colors of the bands are consistent with (c).

In this work, we investigate the evolution of the band gap in the TNS upon the surface adsorption of alkali metal atoms with ARPES and DFT calculations. Alkali adsorbates effectively donate electrons to fill the conduction band of TNS, which in turn gradually closes the band gap gradually. However, our DFT calculation discloses that electron doping is not enough to close the gap, which is instead critically driven by the structural distortion induced by the adsorbates themselves. This work manifests the twofold, structural and electronic, mechanism of the EI phase in TNS. The adsorbate-induced structural distortion has to be considered widely in various surface doping cases.

Results and Discussion

The evolution of the ARPES band dispersion during the growth of an alkali adatom layer on the surface of the TNS is shown in Figure 2 under both polarization conditions. The behaviors are overall similar in both Na and K adsorbates (Supporting Information, Figures S2–S4). In the initial adsorption up to 0.05 monolayer, one can observe the downward (higher binding energy) shift of the valence band. This is obviously due to the charge transfer (electron doping) from alkali adatoms as detailed in the previous ARPES work.35,49 However, a totally different behavior is observed at a higher coverage; the valence-band top shifts in an opposite direction (to a lower binding energy) with the emergence and the gradual increase of an electron pocket at the Fermi energy (Figures2 and 3). At an even higher coverage of above 0.18 ML, the valence band top reaches up to the Fermi energy to be substantially overlapped by the electron pocket. We also note that the characteristic Mexican-hat dispersion of the valence-band top, a hallmark of the EI state, disappears, which indicates the loss of the hybridization between the conduction and valence bands for the EI formation. That is, a substantial reconstruction of the band structure around the Fermi level occurs, beyond the rigid shift due to the electron doping, including the apparent collapse of the EI band gap and the interband hybridization.

Figure 2 Evolution of band dispersions upon surface doping. Evolution of band dispersions measured by ARPES (second derivatives of spectral maps) of TNS during in situ (a, b) Na and (c) K surface doping. The spectra in (a), (b), and (c) are measured by LH and LV polarizations, respectively, which probe Ni 3d and Ta 5d contributions preferentially. The apparent band gap closes down at around 0.15 ML and becomes negative due to the strong overlap of valence and conduction band at a higher coverage. Overall trends of the spectral evolution are consistent for Na and K cases. Dashed lines guide the position of the top (bottom) of the valence (conduction) band.

Figure 3 Two distinct regimes in the electronic structure evolution. Evolution of ARPES normal-emission (BZ-center) spectral intensity upon K dosing, which shows the top (pink) and bottom (red) of valence and conduction bands, respectively. (a) and (b) corresponds to the measurements with LH and LV polarization, respectively. (c) Evolution of the momentum distribution of the ARPES spectral intensity along a direction upon K dosing (up to 0.45 ML), which indicates the occupation of conduction band due to the electron doping. (d) Energy shift of the Ta 4f photoemission spectrum upon K doping (ΔEcore). The spectral intensity for the top of the valence hole band is very weak in (a), as noticed in Figure 2a.

Since the band reconstruction occurs concomitantly with the appearance of the electron pocket, as detailed in Figure 3, its origin would be important to understand the observation. In order to pin down the origin, we performed DFT calculations. We first identify the most stable adsorption site of an alkali adatom as that on top of Ni atomic rows (Figure 1a) among various different possible sites (Supporting Information, Figure S6). This site is confirmed by the scanning tunneling microscopy measurement (Supporting Information, Figure S7). The DFT calculation reproduces well the band structure of the alkali-covered surface, as shown in Figure 4b. The dispersion of the electron pocket follows that of the Ta 5d conduction band without the hybridization but its atomic orbital decomposition exhibits substantial contribution from alkali metal s electrons (Supporting Information, Figure S8). That is, the electron pocket is formed by the donation of s electrons from the alkali metal into the Ta 5d band. This explains the lack of strong polarization dependence of the emerging band in clear deviation from the pristine Ta 5d and Ni 3d bands.

Figure 4 Dual nature of the band gap revealed in DFT calculations. Calculated band structures of (a) pristine and (b) K-adsorbed (0.25 ML) TNS. Red (yellow and blue) color indicates the projection onto the Ta atoms [circled red in the inset of (a)] (Ni atoms with and without K adsorbates circled yellow and blue). The thickness of colored lines is proportional to the weight of the projection on each band. While the band gap is closed on the K-adsorbed chain, the neighboring bare chain still keeps its band gap. (c) Similar calculation with the K-induced structural distortions included but without K adatoms. This calculation represents the effect of the K-induced structural distortion without any charge transfer. (d) Similar calculation for K-adsorbed TNS without any structural relaxation, which represents the charge transfer effect without the structural change. Dashed lines guide the valence and conduction bands to define the band gap, which is indicated by red or blue horizontal bars for positive or negative gaps, respectively.

The origin of the electron pocket immediately tells us that the excitonic band gap is destroyed by the alkali adsorption since the conduction (Ta 5d) and valence (Ni 4d) bands are fully overlapped with their hybridization relieved. We then track in more detail how the band gap collapses by quantitatively measuring the shifts of relevant bands and the occupation of the electron pocket (Figure 3). As mentioned above, the initial rigid shift of the valence band is naturally expected from the gapped nature of the present system. The rigid shift would also move the conduction band toward the Fermi energy and it starts to be occupied from the alkali coverage of 0.05 ML. The EI band gap size at the coverage of 0.05 ML is about 0.15–0.2 eV at maximum since the conduction band minimum is interpolated to reach the Fermi level (Figure 3). This value is in accord with the previous measurements.26,41,50 However, from the coverage of 0.05 ML, reconstruction of the band structure occurs. As shown in Figure 3, the top of the valence band moves upward as the conduction band occupation increases. Thus, the band gap decreases in this coverage regime and closes fully below 0.18 ML. Above this coverage, one can observe the sharp valence band top near the Fermi level, which indicates the diminished interband hybridization after band gap collapse. At a higher coverage, the conduction and valence bands show again the rigid-shift behavior. This tells clearly that the band reconstruction occurs only between 0.05 and 0.18 ML. The three distinct coverage regimes in terms of spectral behavior, below 0.05 ML, between 0.05 and 0.18 ML, and above 0.18 ML, can also be noticed in the energy shift of the Ta 4f electrons, as shown in Figure 3d.

The above result confirms unambiguously the direct relationship between the band gap evolution and metallic electrons since the band restructuring occurs when the system becomes metallic due to the charge transfer from the alkali adsorbate. This observation is incompatible with the trivial semiconducting gap but agrees very well with the excitonic nature of the gap. That is, metallic electrons would readily screen the Coulombic attraction of an electron and a hole to reduce the exciton binding energy, as represented by the band gap in an EI. One major alternative explanation in the present materials is that the band gap also depends critically on the crystal structure. We thus examine in more detail the mechanism of band reconstruction by alkali adsorption through DFT calculation.

To disentangle the structural and electronic contributions, we calculate the band dispersions without alkali adatoms but with only structural distortions induced by the adsorption (Figure 4c). This model includes the structural changes induced by the adsorbates but excludes charge transfer. One can find that valence and conduction bands almost touch each other to drive the system into a zero-gap semiconductor. As mentioned above, the band gap of 0.06 eV in the DFT for the pristine monoclinic TNS is understood to come from the structural changes in large parts and is not directly related to the excitonic interaction. Furthermore, we have checked Se and Ta positions near the K site as detailed in Figures S10 and S11 in the Supporting Information. These data reveal substantial in-plane displacements of the Ta (Se) atoms near the adsorbed sites to transform the local structure into an orthorhombic configuration. This is consistent with the structural origin of the DFT gap, which is closed by the adsorbates. Our experimental data also indicate the structural change. There is a drastic change in the polarization-dependent spectral weight of the Se band at energy away from the band gap region upon the alkali metal adsorptions beyond 0.2 ML (Supporting Information, Figure S12). This must be related to the change in the atomic structure by the adsorbates. However, one can note that the significant part of the gap closure, 0.10–0.24 eV out of the whole gap of 0.16–0.30 eV, and the substantial overlap of the valence and conduction band, about 0.20 eV, (see the experimental result in Figure 3 and the calculated result in Figure 4b) exceed far those explained by the bare structural change only. This clearly indicates that the electronic effect due to the metallic electron occupation constitutes a significant portion of the band structure change observed. We can confirm this scenario further by the DFT calculation without structural changes but with charge transfer from the alkali adsorbate. In Figure 4d, we put the alkali adsorbate on the optimized height decided in the fully relaxed DFT calculation (Figure 4b) but with TNS atomic coordinates fixed in their pristine ones. Here, one can see the rigid shift of the band by the charge transfer, but the band gap is intact, which is consistent with the above discussion on the structural part of the band gap in DFT. We thus conclude that the alkali adsorption dopes the system, which makes the surface layer metallic at a certain coverage, and in turn, this metallic electron greatly reduces the excitonic interaction (gap). At the same time, the alkali adsorbate induces a structural change on the surface layer, which eventually closes down the structural part of the band gap. This result unambiguously indicates the dual nature of the band gap in the present system.

Conclusions

The present work seems to resolve contradictory views of a few recent studies, which emphasize the electronic or structural contributions to the band gap including two recent ARPES studies for the alkali surface dopings.33,35 These two studies showed in principle very consistent experimental results but interpreted them in terms of a dominating electron–hole interaction or a significant lattice effect. In contrast, our present DFT calculations uncover both effects existing. At the same time, our results together with these recent studies demonstrate a way to control the order parameter of an EI effectively. The use of a proximity alkali metallic layer with largely tunable electron density may also be applied to other EIs and also, in principle, to an extensive range of quantum materials systems when the electron density and the screening effect are important for their quantum property. On the other hand, the present work indicates the detailed gap reduction mechanism in a given material must consider the extra structural degree of freedom unavoidable in surface chemical adsorption. To disentangle excitonic and structural effects, it would be helpful to perform gating experiments for thin film samples or to try the surface doping onto Ta2Ni(Se1–xSx)5 samples with the excitonic interaction suppressed.

Methods

ARPES Experiments

ARPES measurements were performed at beamline 4A2 of the Pohang Light Source, which is equipped with a DA-30 electron analyzer (Scienta Omicron). All data were obtained at 70–75 K, and the photon energy was set to 60 eV. Ta2NiSe5 single crystal was cleaved at 70–75 K in a high vacuum (10–11 Torr). The K dose was done in situ using a getter source (SAES).

DFT Calculations

We performed first-principles calculations within the framework of density-functional theory using the Perdew–Burke–Ernzerhof-type generalized gradient approximation for the exchange-correlation functional, as implemented in the Vienna ab initio simulation package.51,52 All the calculations are carried out using the kinetic energy cutoff of 400 eV on 14 × 3 × 4 Monkhorst–Pack k-point mesh for TNS bulk calculations. The spin–orbit coupling effect is included in the self-consistent electronic structure calculation. The lattice constants are taken from experiments,53 but the atoms in the unit cell are fully relaxed with the force cutoff 0.01 Å–1. The electronic self-consistent iteration was converged to 10–5 eV precision of the total energy. To account for electronic correlations in TNS, we employed the DFT+U correction, which captures the Coulomb interaction of Ta 5d and Ni 3d orbitals on the mean-field level, following the simplified (rotational invariant) approach introduced by Dudarev et al.54 The effective Hubbard U (Ueff = U – J) parameters of 2.25 eV (4.75 eV) for the Ta d orbitals (Ni d orbitals) were adopted to accurately represent electronic correlations in accordance with the experimental semiconducting gap of TNS.

Data Availability Statement

The data generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.4c02784.Detailed polarization-dependent ARPES spectral feature, complementary DFT calculations, and atomic structures of K adsorbed TNS in scanning tunneling microscope (PDF)

Supplementary Material

nn4c02784_si_001.pdf

Author Contributions

S.L., J.Y.K., and H.J. performed ARPES experiments. S.L. and H.W.Y. analyzed the data and wrote the manuscript. K.-H.J. performed the DFT calculations. C.I.K. and J.S.K. synthesized the crystal. K.F. took the early version of the ARPES data. H.W.Y. conceived the project idea and extracted the conclusions.

The authors declare no competing financial interest.

Acknowledgments

This work was supported by the Institute for Basic Science (Grant No. IBS-R014-D1). K.-H.J. is supported by the Institute for Basic Science (Grant No. IBS-R014-Y1).
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