Ultrafast electron localization and screening in a transition metal dichalcogenide

2025-04-24 0 0 3.28MB 30 页 10玖币

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Ultrafast electron localization and screening in a transition metal

dichalcogenide

Z. Schumacher1, S. A. Sato2,3, S. Neb1, A. Niedermayr1, L. Gallmann1,*, A. Rubio3,4,

U. Keller1

1Department of Physics, ETH Zürich, 8093 Zürich, Switzerland

2Center for Computational Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8577,

Japan

3Max Planck Institute for the Structure and Dynamics of Matter and Center for Free

Electron Laser Science, 22761 Hamburg, Germany

4Center for Computational Quantum Physics (CCQ), Flatiron Institute, New York 10010,

USA

*Corresponding author. gallmann@phys.ethz.ch

The coupling of light to electrical charge carriers in semiconductors is the foundation of

many technological applications. Attosecond transient absorption spectroscopy measures

simultaneously how excited electrons and the vacancies they leave behind dynamically

react to the applied optical fields. In compound semiconductors, these dynamics can be

probed via any of their atomic constituents. Often, the atomic species forming the

compound contribute comparably to the relevant electronic properties of the material.

One therefore expects to observe similar dynamics, irrespective of the choice of atomic

species via which it is probed. Here, we show in the two-dimensional transition metal

dichalcogenide semiconductor MoSe2, that through a selenium-based transition we

observe charge carriers acting independently from each other, while when probed

through molybdenum, the collective, many-body motion of the carriers dominates. Such

unexpectedly contrasting behavior can be traced back to a strong localization of electrons

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around molybdenum atoms following absorption of light, which modifies the local fields

acting on the carriers. We show that similar behavior in elemental titanium metal1 carries

over to transition metal-containing compounds and is expected to play an essential role

for a wide range of such materials. Knowledge of independent particle and collective

response is essential for fully understanding these materials.

Understanding ultrafast carrier dynamics is important for the engineering of materials towards

applications in electronic and optoelectronic devices. Transition metal dichalcogenides

(TMDCs) have sparked great interest in fundamental and applied science due to the abundance

of observed physical phenomena 2,3. Their unique electronic and mechanical properties,

allowing to go from an indirect bandgap in bulk material to a direct bandgap by exfoliation to

a monolayer, have contributed significantly to their popularity. Many effects, such as the strong

exciton binding energy, the long exciton lifetime and the spin selective valley dynamics have

been studied extensively with optical and electronic probes. Combining different TMDC

monolayers into new heterostructures has introduced further effects such as interlayer excitons

and charge transfer 4-6 and trions in Moiré patterned potentials 7. Besides exfoliation

techniques, which allow for precise control of the thickness and stacking of heterostructures,

progress in chemical vapor deposition (CVD) has enabled the growth of large-area mono and

few-layer samples 8. Some physics aspects, like dark excitonic states 9, exciton generation time

10 and interlayer charge transfer 11,12 are still actively researched due to the difficulty of

characterizing such states. To date most studies focus on the valley, spin, or layer dependent

properties, and only few studies have focused on the element specific response of TMDCs 13-

15.

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Attosecond transient absorption spectroscopy (ATAS) allows for element and carrier specific

probe of excited states dynamics and has become a powerful tool to investigate carrier

dynamics on the few femtosecond scale in semiconductors 16-20 and metals 1,21, and strong field

effects in dielectrics 22,23. In most of these cases, ATAS uses a broadband XUV pulse to probe

the transition between core and excited states with attosecond temporal resolution. With the

probe being resonant with a characteristic core level, ATAS becomes inherently element

specific while its broad bandwidth simultaneously reveals the dynamics of both, pump-excited

electrons and holes.

Here, we apply ATAS to a few-layer CVD grown 2H MoSe2 (6Carbon Inc.) with a bandgap of

≈1.55 eV. Surprisingly, we find qualitatively different dynamics in both conduction and

valence band for probe transitions originating from either Mo or Se. This is remarkable because

the band structure in the vicinity of the bandgap is formed by covalent bonds between the d-

orbitals from Mo and the p-orbitals from Se with the two contributing to the density of states

with similar magnitude. Such a qualitative difference in response was previously observed in

the similar material MoTe2 but not explained 13. In a recent publication, contrasting behavior

observed for two W core level transitions in WS2 was suggested to originate from different

degrees of localization of the initial states 24. In contrast, by combining our observed transient

spectral features with ab initio calculations, we are able to attribute the qualitative difference

to pump-induced real-space carrier localization into d-orbitals of the transition metal and the

resulting local screening modification. Interestingly, the Se response remains entirely

unaffected by such many-body dynamics. In MoSe2 we find the localization effects to last

longer than in the elemental transition metal Ti and suggest that this faster decay is due to the

much higher electron-electron scattering rates in the metal compared to a material with a

bandgap.

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We simultaneously probe the core level transitions from Mo 4p states and the spin-orbit split

Se 3d states to the valence and conduction band (VB/CB) (Fig. 1). For multilayer MoSe2 the

maximum of the valence band is located at the G-point and has mostly 4d and 4p-orbital

contributions from Mo d3z2-r2 and Se pz orbitals, respectively. The orbital character of the

conduction band minimum originates predominantly from dxy and dx2-y2 of Mo and px, py orbital

contributions of Se. Furthermore, a non-negligible d3z2-r2 and pz orbital contribution from Mo

and Se atoms, respectively, is present in the conduction band 25-27. The strength of the d and p-

orbital contributions of the two respective atoms to the covalent bonds forming the band

structure around the bandgap is comparable as shown in our calculated projected density of

state (PDOS) (Fig. 1c).

Fig.1. a) Experimental setup. A near infrared (NIR) pump and XUV probe pulse are delayed in time. The

residual NIR is eliminated by an aluminum filter before the transmitted XUV radiation is measured in a

spectrometer. b) Total density of states of bulk MoSe2 calculated by density functional theory. The NIR pump

excites carriers across the Fermi level, while the XUV probes the transitions from the Mo 4p and Se 3d core

levels to the valence and conduction band (spin-orbit splitting of Se 3d not shown). c) Projected density of

states showing dominant contributions of Mo 4d (purple solid line) and Se 4p (green solid line) states to the

valence and conduction bands. Mo 4p and Mo 5s have been multiplied by a factor for better visibility.

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Our few-layer MoSe2 sample was transferred onto a 30 nm thin silicon nitride membrane as a

substrate (Ted Pella) with excellent transparency in the energy window of our experiment. The

broadband XUV probe spectrum from an attosecond pulse train spans photon energies from 30

to 70 eV. We use a 10-fs near infrared (NIR) excitation pulse with a center photon energy of

1.54 eV and a peak intensity of ≈ 6 ∙ 10!! W cm"

⁄ at 1 kHz pulse repetition rate to promote

carriers across the bandgap with a density of ≈ 2 ∙ 10"! cm#

⁄. The relatively high pump-

induced carrier density can be treated as an electron-hole liquid since it is above the Mott-

transition (see SI). The interferometric ATAS setup housed in vacuum is described in more

detail elsewhere 28.

Spectral response

The pump-induced transient changes in XUV absorption are shown in Figure 2a as a function

of time delay between the NIR pump and the broadband XUV probe pulse. Lineouts of that

data integrated over a time span of ±20 fs for several time delays are plotted in Figure 2b. The

different core level energies allow us to separate the element specific response with the probing

photon energy. The Mo response covers the XUV photon energy range between 32 to 50 eV

with the strongest signal for the Mo 4p core level to valence and conduction band transition

(32 – 43 eV) and some weaker excitations into higher-lying and delocalized states above 47

eV. Of these, the 5s states contribute the most among the bound states (see SI). The Se response

for the XUV photon energy range of 54 to 60 eV shows two sharp alternating bands with

increase and decrease in absorption that resemble the derivative of an absorption peak in shape.

The double appearance of this derivative-shaped feature arises from the spin-orbit split Se

3# "

⁄ and 3! "

⁄ transitions with an energy difference of ~1 eV (see SI). In comparison the Mo

specific signal in the valence and conduction band is more broadband and positive. As is shown

below, this drastically contrasting behavior is not a probe-induced effect. Due to the mixed Mo

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摘要：

-1-UltrafastelectronlocalizationandscreeninginatransitionmetaldichalcogenideZ.Schumacher1,S.A.Sato2,3,S.Neb1,A.Niedermayr1,L.Gallmann1,*,A.Rubio3,4,U.Keller11DepartmentofPhysics,ETHZürich,8093Zürich,Switzerland2CenterforComputationalSciences,UniversityofTsukuba,Tsukuba,Ibaraki305-8577,Japan3MaxPlanc...

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Ultrafast electron localization and screening in a transition metal dichalcogenide

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