# Nitrogen induced defects in VSe 2 studied by scanning tunneling microscopy

U Chazarin, M Lezoualc, W W Pai, R Sankar, C Chacon, Y Girard, César

González, Y J Dappe, J Lagoute

► To cite this version:

U Chazarin, M Lezoualc, W W Pai, R Sankar, C Chacon, et al. Nitrogen induced defects in VSe 2 studied by scanning tunneling microscopy. Journal of Physical Chemistry C, 2025, 10.1021/acs.jpcc.5c05337. [DOI: https://doi.org/10.1021/acs.jpcc.5c05337](https://doi.org/10.1021/acs.jpcc.5c05337)

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Distributed under a Creative Commons Attribution 4.0 International License
Nitrogen Induced Defects in VSe₂ Studied by
Scanning Tunneling Microscopy

U. Chazarin,† M. Lezoualc'h,¶ W. W. Pai,*,‡ R. Sankar,§ C. Chacon,† Y. Girard,†

César González,∥,⊥ A. Smogunov,¶ Y. J. Dappe,# and J. Lagoute*,†

†Université Paris Cité, Laboratoire Matériaux et Phénomènes Quantiques, CNRS,
F-75013, Paris, France

‡Center for Condensed Matter Science (CCMS), National Taiwan University, 11106
Taipei, Taiwan

¶SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex,
France

§Institute of Physics, Academia Sinica, Taipei 11529, Taiwan

∥Departamento de Física de Materiales, Universidad Complutense de Madrid, E-28040
Madrid, Spain

⊥Instituto de Magnetismo Aplicado UCM-ADIF, Vía de Servicio A-6, 900, E-28232 Las
Rozas de Madrid, Spain

#Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, 91120,
Palaiseau, France

E-mail: Corresponding_author:wpai@ntu.edu.; jerome.lagoute@univ-paris-diderot.fr
Abstract

Atomic-scale studies of dopants in transition metal dichalcogenides (TMD) are im-
portant for tailoring their physical and chemical properties, and provide a clearer pic-
ture of structure-property correlation. Nitrogen atom as a dopant in TMD materials
promises tuning of carrier type and enhanced catalytic properties, however experimen-
tal studies have rarely addressed its atomic-scale details. Here we present a study of
nitrogen dopants in nitrogen-plasma-treated 1T-phase VSe₂ bulk crystal by scanning
tunneling microscopy (STM) and spectroscopy (STS). Three main N-induced species
are classified. One type (type-I) is shown to be substitutional N atoms at the bottom
layer Se sites. Type-II has a not-yet-determined structure. The final type-III N atom
is an adsorbed N atom at quasi-bridge sites between Se atoms. This N atom specie
exhibits distinct tip-induced dynamical motion. Density functional theory (DFT) cal-
culations corroborate with experimental STM and STS features, lending support of
the proposed models. These results highlight the great varieties of even simple atomic
dopants in TMD materials and the needs to finely control their properties for future
applications.

Keywords: Scanning Tunneling Microscopy, Density Functional Theory, Transition Metal Dichalcogenides, VSe₂, Nitrogen doping, defect study

**Introduction**

Defects play a crucial role in materials properties. This is expected to be even more pro-
nounced in the case of two-dimensional (2D) materials where reduced dimensionality may
amplify their influence. Consequently, defect engineering has emerged as a promising route
to render novel functionalities in 2D materials.¹ Leveraging the properties of defects requires
deep understanding of their geometric and electronic structures, and such knowledge remains
much unexplored.
Much attention has been directed to the role of defects in transition metal dichalcogenides (TMDs)2-5 since such materials can exhibit a wide variety of ground states from simple semiconductors to metals, to the emergence of charge density waves (CDW) or superconductivity. In semiconductor compounds, such as MoS2, defects can change the band gap width,4 structure, and properties.6,7 In metallic TMDs, defects can interact with CDW as pinning centers, influencing the stability and physical properties of the CDW.8,9 Defects can often carry charges and thereby modulate the host materials properties, or generate reactive sites on their own - a feature that is particularly relevant for catalytic applications. For instance, in VSe2 monolayer, vanadium vacancies have been identified as high-performance electrocatalytic center for both oxygen evolution and reduction reactions.10 Hence, engineering such defects through the introduction of foreign species may provide a way to further improve catalytic efficiency.

Chemical substitution constitutes de facto a way to introduce dopants in many materials. Among various dopant choices for 2D materials, nitrogen has been extensively used to modify the electronic properties of 2D materials. In graphene, nitrogen doping typically results in the substitution of carbon atoms with nitrogen, leading to n-type doping¹¹ and various local N bonding configurations. Nitrogen-induced states in graphene have been widely observed and characterized in previous STM/STS measurements.¹²,¹³ In TMDs, nitrogen can potentially substitute chalcogen atoms, either in the top or bottom chalcogen (X) layers in the MX2 (M:metal) TMD layer. The proximity of N to the chalcogen group in the periodic table² allows N atoms to substitute chalcogen atoms. In semiconductor TMDs, nitrogen dopants can often create acceptor levels just above the valence band, leading to p-type doping which is uncommon in pristine natural TMDs. More complicated dopant structures might further distort lattice and create new kinds of electronic states. Nitrogen plasma-induced defects could also locally disrupt the CDW order, as it has been reported in TiSe2.¹⁴

Here, we investigate nitrogen incorporation into VSe₂. Using a combination of N-plasma
processing, STM/STS characterization, and *ab initio* DFT calculations. A variety of N-
induced point defects are identified and the majority can be classified into three differ-
ent types. Geometric structures are proposed through comparison between atomic-scale
STS/STM results and DFT modelling. As a result, we discovered surprisingly that substitu-
tional N atoms prefer embedded Se sites rather than Se sites at the surface, while adsorbed N
atoms show tip-induced confined dynamics of motion. These findings highlight the complex
nature of nitrogen incorporation and point to rich opportunities for further exploration of
dopant configurations in VSe₂.

Methods

Bulk T-phase VSe₂ crystals were grown by Chemical Vapor Transport (CVT) method and has been also used in previous works. ¹⁵ A 1-2 mm thick sample was glued onto a sample plate, and was cleaved under ultra-high vacuum (UHV) condition (1 × 10⁻⁹ mBar) to obtain a contamination-free surface. The sample was exposed to a nitrogen flux under UHV produced by a UHV-compatible RF plasma source (from Oxford Applied Research) fed with N₂ at a power of 100W. A voltage of 1 kV was applied between deflection plates at the exit of the plasma source to prevent ions from impinging on the sample, so that only N radicals interact with its surface. During N-plasma treatment, the chamber pressure is about 7 × 10⁻⁴ mbar, the deposition time is around 20 s, and the sample is electrically ground and kept at room temperature. The sample was then directly transferred to the low temperature STM chamber at a pressure of 2 × 10⁻¹¹ mBar. STM/STS measurements were performed at a base temperature of around 4.6 K. STM topography images were obtained in constant current mode. Tungsten tips were home-made with electrochemical etching. STS measurements were conducted with lock-in detection using a modulation frequency of 746 Hz and a r.m.s. amplitude of 20 mV.

Density Functional Theory (DFT) calculations were performed using the localized-orbital
molecular-dynamics code as implemented in FIREBALL.¹⁶⁻¹⁹ This approach was employed to
optimize and calculate the electronic structure of various defects (N adatom, substitutions,
and vacancies) in a (8 × 8) free-standing VSe₂ superstructure. The structural optimizations
were carried out until the forces on all atoms were reduced below 0.1 eV/Å. A Monkhorst-
Pack grid of 4 × 4 × 1 k-points was employed for both structural optimization and density
of states (DOS) calculations. Simulations of scanning tunneling microscopy (STM) images
were performed following a well-established Keldysh-Green formalism fully detailed in Ref.²⁰

Results

Nitrogen plasma doped VSe₂

A bulk crystal was cleaved in UHV, following the procedure detailed in our previous report,¹⁵ and subsequently exposed to a nitrogen plasma source (see methods). STM surface image taken at 4.6 K after N₂ plasma treatment (see the scheme in Figure 1) is shown in Figure 1, revealing different types of defects as described in more details below. The expected (4 × 4) CDW of bulk VSe₂ is also observed, identical to the one of pristine VSe₂ (See supporting informations Figure S1). We have identified several distinct types of defects with a majority that can be classified into three types, referred hereafter as type-I, type-II, and type-III. In the STM image measured at -1 V, Type I nitrogen appears as a protrusion (~8 pm), Type II appears as a hole (~6 pm) and Type III appears much higher (~60 pm) and noisy than the other two N types, and noisy (see the inset in Figure 1). The characteristics of these defects, that will be discussed in more details below, are different from native defects of VSe₂ (See supporting informations Figure S2). Less frequent N-induced defects were also observed but they occur too sporadically for systematic and accurate classification. These rare species may originate from more complex structural rearrangements or from impurities introduced during plasma exposure. From a statistical analysis of 87 defects observed in various images,
we identified 64% as Type I, 26% as Type II, and 6% as Type III, with the remaining defects unassigned. (See Figure S3 for a large scale image showing the distribution of different defect types.) In the following, we therefore focus on the three predominant nitrogen types and named them type-I,II,III defects.

<figure><img src="image_2.png" /><figcaption>Figure 1: Schematic view of a VSe₂ sample exposed to a flux of nitrogen atoms produced by a nitrogen plasma source. 3D view of filtered STM image containing the three types of nitrogen defects (V = -1V, I = 10pA, 10×10 nm²). The inset shows the STM image raw data.</figcaption></figure>

## Type I defect

In Figs.2(a, b) we show STM images measured at negative (-1 V) and positive (+1 V) polarities, of a sample area containing seven type-I defects (one is marked by a red arrow) and a type-II defect in the middle of image. The type-I defect appears as a bright spot at negative bias and a dark hole at positive bias, while type-II defect presents the opposite behaviour. Topographic profiles crossing the type-I,II defects at both polarities are shown as insets in Figs.2(a, b). Type-I defects presents a notably higher spatial density as compared to the other two defect types. CDW still appears in the vicinity of the defects, indicating
that they do not induce significant perturbation to the CDW structure.

<figure><img src="image_3.png" /><figcaption>Figure 2: (a - b) STM topography image of the type I and type II defects at -1V (a) and +1V (b) (I = 200 pA). The linescan taken on the two nitrogen type defects are shown in inset for type I (white) and type II (blue). (c) dI/dV spectrum of the type I defect compared with the pristine VSe₂ spectrum away from the defects. The dotted line indicates the Fermi level. 7 spectra on VSe₂ (light black) and 4 spectra on Type I defect (light red) are shown, together with the average spectra (black and red curves). (d - e) DFT simulated STM topographic images of a N substituted Se bottom layer defect in a VSe₂ cell at respectively V= -1V and V= +1V. (f) Calculated DOS for the pristine VSe₂ and the N substituted Se bottom layer defect projected on the three top layer Se atoms surrounding the defect site.</figcaption></figure>

The dI/dV spectroscopy right atop the type-I defect (Figure 2(c)), resembles that of the
pristine VSe₂ (corresponding to previously reported spectra<sup>21,22</sup>) but with peak shifts similar
to what was found for Na-doped VSe₂,<sup>15</sup> i.e., peak at -0.06 V shifts downward and peak at
0.75 V shifts upward. To identify the nature of this type-I defect, we performed DFT cal-
culations on several configurations. We considered (1) top and (2) bottom Se vacancies; (3)
nitrogen substitution of V atom; (4) N substitution of top and (5) of bottom Se atoms; more
complex defects were also calculated, (6) 3 top Se vacancies and (7) 3 nitrogen substitution of top Se. Among all the seven STM-calculated images, only those in which an N atom substitutes a bottom Se atom successfully are able to reproduce the experimental observations for type-I defect, i.e., a bright spot at negative energy and a dark spot at positive energy (see Figure 2(d-e) and supplementary informations Figure S4). In addition to the calculated STM images, calculated partial DOS (Figure 2(f)) features are consistent with experimental observation - a slight downshift of the peak near the Fermi level (arising from vanadium *d* state) compared to pristine VSe₂, and an upward shift for the peak at 0.75 V. We also found a charge transfer of 0.03 electrons to the three V atoms surrounding the N dopant, including 0.02 electrons localized on the *d* orbitals. This charge redistribution is consistent with the observed downshift of the *d*-state peak near the Fermi level. Therefore, a type I defect may correspond to a nitrogen substitution on Se bottom atom. For the excluded configurations (1-3), their STM images are shown in supplementary information. The fact that nitrogen goes to the bottom layer may be related to the small size of N combined with its kinetic energy gained in the plasma source.

## Type II defect

The type-II defect, which appears dark at negative bias and bright at positive bias, is shown in Figure 3(a,b). The dI/dV spectrum in Figure 3(c) retains the general characteristics of pristine VSe₂ surface. At positive bias specifically, a broad DOS is observed, consisting in a peak at 1.0 V and a shoulder at 0.7 V. These two peaks have different orbital nature, as shown in reference.¹⁵ The peak at 1.0 V is of 3d<sub>z²</sub> orbital (where z is the out-of-plane direction) and it is enhanced for type-II defect. Close to the Fermi level, new spectral features develops and form three distinct peaks at approximately -0.05 V, 0.04 V, and 0.22 V (Figure 3(c)). Since these peaks are also mainly of 3d<sub>z²</sub> orbital character, it suggests that this type-II defect interacts more strongly with the V 3d<sub>z²</sub> state. Since the structure
of this defect is not fully captured by DFT calculations performed on a set of simplified
configurations, the exact atomic structure of this defect remains to be determined. Images
taken to show bias-dependent defect contrast do not usually come with substrate atomic
resolution. An atomic resolution image of type-II defect is shown in supporting information
Figure S5. A defect contrast spreading over several atomic sites is visualized, presumably
due to the interplay of structural and electronic effects in STM images.

<figure><img src="image_4.png" /><figcaption>Figure 3: (a - b) STM topography image of the type II defect, respectively negative and positive bias (V = -1V, I = 2.5nA, and V = +1V, I = 200pA) (c), STS spectrum of the type II defect (red) and the pristine VSe₂ (black).</figcaption></figure>

Type III defect - N atoms on VSe₂ surface

The third type of nitrogen induced defect observed is shown in Figure 4. This defect appears noisy (Figure 4(a)) in a standard STM image and has a large apparent width (diameter of about 1 nm). The bias contrast of this type-III defect remains unchanged with bias polarities and appears bright, in contrast to behaviors of the other two defect types discussed above. The "frizzle" and "noisy" image of this defect type is related to an apparent mobility of this specie. This is evidenced in the STS data, as three telegraph-like tunneling levels are observed when the STS is taken near -but not at - the center of the noisy defect, as discussed below. On one hand, the DOS peak crossing the Fermi level shows no significant shift (Figure 4(b)).
In contrast, a prominent DOS peak at 1.1 V and a gentle shoulder at 0.7 V were observed. These features are similar to that of type-II defect, except that the peak near Fermi energy shows different degrees of shift.

While this defect appears noisy and mobile, it is possible to characterize the actual defect position using a higher scanning imaging rate. A series of such zoom-in defect images shows in Figure 4(d-f) that the defect position switches between 3 specific positions (see also the movie in supporting information). In Figure 4(d-f), each measured position (in 85 images) is marked by a grey dot and all grey dots form three groups, with their respective average positions marked by red dots. The distances between the three averaged positions are measured as 0.20 nm, 0.18 nm and 0.11 nm. Given the average measured inter-site distance of 0.16 nm, it corresponds closely to half the in-plane lattice constant of VSe₂ (0.336 nm). This suggests that the N adatom is located near a bridge position between two Se atoms (see the red crosses on Figure 4(c)). The observed noise of this type-III defect is thus related to a switching of its position between three adjacent bridge sites. To investigate this switching further, we measured the time dependence of the tunneling current with the STM tip located close to the defect. Three distinct current levels (low:50, middle:100, high:200 pA) are clearly observed, as shown in Figure 4(g). From these values we can numerically extract the switching yield for each level, as defined by the number of switching events per electron per unit time. This is given by $\eta_{\alpha} = \frac{e}{I_{\alpha}\tau_{\alpha}}$ where e is the electron charge, $I_{\alpha}$ is the tunneling current intensity associated to the level $\alpha$, and $\tau_{\alpha}$ is the total duration of the level in the signal. To compute these yields accurately, we acquired data all at a fixed tip position for all measurements, with minimum thermal drift. This approach makes it possible to clearly define the low, median, and high current levels, corresponding respectively to the tunneling current when the adatom occupies each of the three accessible positions. We then measured the yields associated with these three positions as a function of tunneling current (See Figure 4(h)). The results, however, showed no consistent monotonic trend across the
<figure><img src="image_5.png" /><figcaption>Figure 4: (a) STM topographic image of the type III defect appearing in the center of the image (V=-0.1V, I=200pA) (b) STS spectra taken on the pristine VSe₂ area and on the type III defect. (c) Representative scheme of the adatom positions with respect to the underlying Se atoms (in blue), and the associated expected distances. (d-f) STM topographic image of the adatom in the three different adsorption sites. The positions measured on a series of successive images are indicated by black dots, the average positions of the three sites are indicated in red. (V = -50mV, I = 10 pA), (g) Tunneling current acquired at 0.2V bias as a function of time (black) and the fitted telegraph signal (red). (h) Mean yield as a function of set point current for the three levels. (i) Mean yield as a function of bias for different levels.</figcaption></figure>
full set of data. This suggests that the switching is due to the tip and is an one-electron
process.²³⁻²⁶ Furthermore, yields as a function of bias for the three different levels are shown
in Figure 4(i). For all three levels, the curves show a similar shape with symmetric thresholds
at +0.5 V and -0.5 V with respect to the Fermi level. This is indicative of an inelastic process
for the switching mechanism with a threshold of 0.5 eV.

The N adatom jumps only between three bridge sites centered around a hollow site, see
Figure 4(c). Why does not the N adatom jump to other bridge sites around the adjacent
hollow site? One possibility is that the N adatom is not exactly at the bridge site but
asymmetrically closer to the Se triangle center, thus forming a potential barrier for the N
atom to jump long distances. We thus performed DFT calculations with a 8 × 8 1T-VSe₂
supercell to investigate the energetics of different potential N adatom configurations. The
considered N adatom sites include (1) a Se triangle hollow site atop a V atom(Figure 5(a));
(2) a Se triangle hollow site atop a Se atom Figure 5(b)), and (3) a Se-Se bridge site (Figure
5(c)). Interestingly, after optimizing the structure models, we found that N adatom atop a V
atom is unstable and tends to shift to a bridge configuration, similarly to configuration (3).
On the other hand, the N atom structure (2) remains stable. However, its adsorption energy
is less favorable than the bridge configuration (3), which is the most stable by approximately
22.3 meV per unit cell.

These above DFT results are in good agreement with experimental observations, which
show a preference for the quasi-bridge configuration for the N adatom. We also computed
the projected Density of States (PDOS) of this configuration, with PDOS integrated at those
circled red atoms as shown in Figure 5(b) and (c). The PDOS results are plotted alongside
the PDOS of the pristine VSe₂ projected on a single Se atom (Figure 5(d,e)). In both
cases, we observe a downward shift of the peak near the Fermi energy in calculations. This
nonetheless contradicts the experimental spectra (Figure 4(b)) in which the peak intensity is
reduced but remains at the Fermi energy. In the calculations, however, the effect of a single
<figure><img src="image_6.png" /><figcaption>Figure 5: Different potential atomic configurations of an N adatom to model type III defects: (a) hollow site on top of V atom, (b) hollow site on top of Se atom, and (c) bridge site. Only the atomic configurations in (b) and (c) remain stable. (d) and (e) respective (b) and (c) projected density of states (PDOS). The DOS is projected on the orbitals of the atoms circled in red in (b) and (c).</figcaption></figure>

defect atom may be amplified by the use of a small periodic supercell, as compared to the experiment where the same defect corresponds to a much lower concentration embedded in the bulk environment. For the higher energy peak at 1 V, the PDOS in the quasi-bridge configuration better matches experimental observations, showing a broadening of the peak.

Conclusion

We have studied nitrogen plasma-induced defects in bulk $VSe_2$ by combining STM/STS and DFT. We identified three main types of defects. Type-I defects are consistent with nitrogen substitution at bottom Se sites. Type-II defects likely involve more complex structures
and is not yet fully resolved. Type-III defects, characterized by their apparent mobility, give
strong evidence that such N adatoms are occupying quasi-bridge sites. These N adatoms can
dynamically switch between adjacent but confined adsorption sites through STM tip-excited
inelastic energy transfer process. These results provide new understanding of nitrogen defect
properties in a TMD (VSe₂) but it could also offer insights for other TMDs and for future
defect engineering in 2D materials.

## Acknowledgements

We thank the French National Research Agency (ANR-20-CE09-0023 DEFINE2D project) for financial support. We thank the CNRS and MOST (grant 114-2927-I-002-523) for financial support through the France-Taïwan IRP project ENTER-2D. WWP acknowledges support from Taiwan MOST DEFINE2D (111-2923-M-002-011-) and 110-2112-M-002-040-MY3 funds for support. R.S. acknowledges the financial support provided by the Ministry of Science and Technology in Taiwan under project number NSTC-114-2124-M-001-009, NSTC-113-2112-M-001-045-MY3, and Academia Sinica for the budget of AS-iMATE-114-12.

## Supporting Information Available

Atomic resolution image of pristine VSe₂; Native defects in VSe₂; Large scale STM image of defect distribution; DFT calculations on various defects; Atomic resolution image of N-defect in VSe₂; Movie showing a series of consecutive images of an adatom switching between 3 positions (1.5nm × 1.5 nm).
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