Abstract
Two-dimensional (2D) semiconductors are one of the most extensively studied modern materials showing potentials in large spectrum of applications from electronics/optoelectronics to photocatalysis and CO2 reduction. These materials possess astonishing optical, electronic, and mechanical properties, which are different from their bulk counterparts. Due to strong dielectric screening, local heterogeneities such as edges, grain boundaries, defects, strain, doping, chemical bonding, and molecular orientation dictate their physical properties to a great extent. Therefore, there is a growing demand of probing such heterogeneities and their effects on the physical properties of 2D semiconductors on site in a label-free and non-destructive way. Tip-enhanced Raman spectroscopy (TERS), which combines the merits of both scanning probe microscopy and Raman spectroscopy, has experienced tremendous progress since its introduction in the early 2000s and is capable of local spectroscopic investigation with (sub-) nanometer spatial resolution. Introducing this technique to 2D semiconductors not only enables us to understand the effects of local heterogeneities, it can also provide new insights opening the door for novel quantum mechanical applications. This book chapter sheds light on the recent progress of local spectroscopic investigation and chemical imaging of 2D semiconductors using TERS. It also provides a basic discussion of Raman selection rules of 2D semiconductors important to understand TERS results. Finally, a brief outlook regarding the potential of TERS in the field of 2D semiconductors is provided.
Keywords
- TERS
- Raman spectroscopy
- 2D semiconductors
- TMDC
- MMC
- plasmonics
- nanoscale chemical imaging
- local heterogeneities
1. Introduction
The ability of isolating stable, atomically thin monolayers of layered materials stimulated a new field of atomic-scale interface physics, with tremendous potential for novel quantum optoelectronic applications [1]. Graphene, the first isolated 2D material consisting of an atomically thin carbon sheet provides much stronger mechanical strength and electrical or heat conductivity compared to its bulk counterpart graphite [2, 3, 4]. Thus, graphene was predicted to have great impact on devices with ballistic charge transport or quantum anomalous Hall effect [5]. However, the lack of a bandgap coupled with challenges and consequences associated to the attempts of bandgap opening hinders its application as an active material in semiconducting devices [6]. In this context, 2D semiconductors have attracted significant attention owing to their suitable bandgap required for optoelectronic applications. Some of these semiconductors already possess carrier mobilities that can outperform existing Si CMOS technology at the scaling limit, although they are affected by some other limiting factors [7]. Therefore, extensive research is going on to gain both fundamental understanding of these materials and to explore new 2D semiconductors for potential applications.
One of the novel aspects of 2D semiconductors is that their optical properties can be tuned as a function of layer number [8]. The most famous 2D semiconductors in this respect are the transition metal dichalcogenides (TMDCs). For example, in bulk TMDCs are indirect bandgap semiconductors, which can be tuned upward ∼1 eV with decreasing layer number down to monolayer. More importantly, the nature of the bandgap also changes from indirect to direct at the monolayer thickness [9]. Density functional theory (DFT) predicts that the direct excitonic transition energy at the Brillouin zone (BZ) K point in TMDCs hardly shows any dependence with respect to the layer thickness [9]. However, the indirect transition along the
Even though these semiconductors are few atoms thick, some of them can absorb up to 15% of light in the visible range due to strong light-matter interaction [13]. The dramatically reduced dielectric screening in the out-of-plane direction creates strongly bound excitons with binding energies in the range of few hundreds of meV [14, 15]. Therefore, their optical response is dominated by excitons or multi-particle complexes at room temperature (RT) [16, 17]. Atomically flat dangling bond free surfaces of these materials are free from career scattering caused by surface roughness, which can lead to high performance optoelectronic devices with large on–off ratio (∼108), photoresponsivity (∼ 107 mAW−1), and career mobility (103 cm2V−1 s−1) at RT [7, 18, 19]. Both few layer BP and InSe based field effect transistors show comparable career mobility (103 cm2V−1 s−1) at RT suggesting they can outperform Si based CMOS at the scaling limits [20]. However, BP suffers from poor stability in air [21] and the polar nature of InSe leads to layer dependent Fröhlich interaction [22] and thus optical phonon limited career mobility.
For all the extraordinary optoelectronic properties of 2D semiconductors, the influence of local heterogeneities such as edges, grain boundaries, defects, strain, doping, chemical bonding
2. Principle of TERS
Raman scattering is at the core of TERS, which is light inelastically scattered by elementary excitations such as vibrational modes (phonons) in the material probed. Consequently photons are emitted that are shifted in energy with respect to the energy of the exciting light. The shifts are directly correlated to the phonon frequencies of the material. This effect was first observed by C. V. Raman in 1928 and thus named after him [27]. The fundamentals and the various aspects of Raman scattering are comprehensively covered
TERS works using the same principal as SERS,
The lightning rod effect is introduced by the anisotropy of the tip geometry (the larger the anisotropy, the stronger the field enhancement) and thus independent of the excitation wavelength. However, the LSPR is created due to the collective oscillation of conduction band electrons at the metal surface. Therefore, several factors influence the LSPR energy and oscillator strength, such as material, size, shape, and dielectric interface [36, 39, 40]. Gold and silver are the two most widely used noble metals for TERS tips. Both metals reveal small dielectric loss, stability in air, and tunability of the LSPR in the visible spectrum. Importantly, they both have some advantages and disadvantages, which become critical deciding factors for TERS applications. For example, silver shows a more pronounced plasmonic effect but gold provides a better environmental stability. Thus, the latter makes gold the more popular material for TERS tips. Additionally, for the same size and shape of the apex, the LSPR of a tip made of Ag lies at a higher energy than that of an Au tip [36]. Therefore, Ag tips are more commonly in use for green excitation and Au tips are more suitable for red and near-IR TERS applications.
The TERS schematic shown in Figure 1a is also known as conventional TERS, in which the local electric field is created by the tip dipole oscillation. The TERS sensitivity (
Figure 2a displays the schematic of the gap-mode TERS configuration. The strength of the dimer coupling and consequently the gap-mode TERS sensitivity depend on the tip-substrate distance
For a certain tip-substrate geometry both
For the TERS intensity Eq. (2) becomes
At
For a very small
A more detailed theoretical study of the TERS enhancement and spatial resolution in gap-mode TERS as a function of tip diameter is shown in Figure 2f–g. The tip-substrate distance
3. Surface selection rules of TERS in 2D semiconductors
According to the FEM simulations, the overall electric field intensity beneath the tip apex is zero for an incident beam polarized along the XY plane as shown in Figure 3a. Therefore, for an incident beam polarized along the
The above enhancement condition is also applicable for the gap-mode TERS configuration. Hence, for a side-illumination geometry at an incident angle, which satisfies the tip polarization condition, a tip in gap-mode TERS configuration confines and enhances the
The idea of surface selection rules was first conceptualized by Moskovits on flat metallic surfaces [45]. Due to the adsorption of molecules on a flat metal surface, they will experience a different electric field, since incident light is reflected off the metal substrate. Assuming both
The primes are associated to the scattered radiation,
According to Eq. (10) the effective Raman scattering depends on the
In a real situation, the normal to the 2D material basal plane is parallel to the direction of
It is also worth noting that above mentioned assumption is a simplified model, which does not consider interface chemistry. For example, in the case of chemisorption the molecular geometry or orientation may alter significantly leading to the breaking of the Raman selection rules. In such cases, some Raman inactive modes can become Raman active [46]. Moreover, light-plasmon coupling in a nanocavity between the tip and the metal substrate can lead to the alteration of classical Raman selection due to photon tunneling through perturbation of the evanescent field [47].
In polarization dependent Raman measurements we observe the scattered light intensity as a function of polarization directions of both the incident and scattered light. The Raman intensity can be written as
Here,
There is one more doubly degenerate Raman mode with
Another important aspect of TERS selection rules is demonstrated by the enhancement of the infra-red (IR) active out-of-plane
4. Local phonon properties in 2D semiconductors
4.1 Strain induced local bandgap modulation
Strain plays an important role in manipulating optoelectronic properties of 2D semiconductors. The remarkable strength of 2D semiconductors such as TMDCs compared to other conventional semiconductors makes them well suited for straintronic applications. For example, a MoS2 monolayer can sustain a large biaxial strain >10%; whereas bulk silicon usually breaks at a strain of ∼1.5% [53]. This extraordinary property of 2D semiconductors has led to burgeoning research of strain engineered optoelectronic properties in recent times [53, 54, 55]. Among them, most of the studies provide macro/microscale information of strain induced optoelectronic manipulation using conventional optical spectroscopy. In contrast, the local modification of band structures due to spatially inhomogeneous strain distribution on the nanoscale is much less explored. Like the band structure, phonons in 2D semiconductors are directly affected by a wide range of parameters including strain [56, 57]. Therefore, Raman spectroscopy, one of the most frequently used non-destructive spectroscopies, becomes a powerful technique to investigate the different components of strain and their effect on optoelectronic properties of these semiconductors. Due to energy and momentum conservation, phonons participating in first order Raman scattering processes need to satisfy the wavevector condition
The two well-studied first order Raman modes in TMDCs are in-plane
It is important to note that the above described TERS experiments were performed in resonant Raman condition with 638 nm excitation (excitation close to the
The rich variety of second order Raman features in MoS2 in resonant Raman condition also yields important information about its physical properties and the electronic band structure. Zhang
Like externally induced strain via transferring TMDCs monolayers on nanostructured substrates as discussed above, direct evaporation of a metal, for example gold on monolayer MoS2, can cause large strain (∼ 5%) at the interface due to lattice mismatch. Combining various SPM techniques and TERS Jo
TERS has become a powerful technique to probe local strain variation in 2D semiconductors, which in turn helps us to understand the inhomogeneities in optoelectronic response of these materials.
4.2 Effect of variance in atomic registry on electronic properties
The reduced dimensionality of 2D semiconductors makes them susceptible to structural variations near the internal and external boundaries. Parameters, such as impurities, defects, and grain boundaries interfere strongly with intrinsic properties resulting in highly inhomogeneous optoelectronic response [23, 69, 70]. While conventional optical spectroscopy leaves a confusing picture regarding the effect of these heterogeneities [71], near-field optical studies can help us to understand the correlation of local structural heterogeneities with nano-optical response. In recent years, the combination of TEPL and TERS has been successfully introduced to investigate these heterogeneities. In TMDCs, the out-of-plane
Due to their extraordinary physical properties, many groups are now synthesizing 2D semiconductors by CVD and other deposition techniques with the vision of industry scale production. However, in terms of crystal quality these deposition processes still require further optimization to be able to use them in electronic applications. CVD grown 2D semiconductors often reveal several structural heterogeneities as discussed in this section. Therefore, proper understanding is the key for the optimization of the deposition process and hence improving the crystal quality. While confocal micro-Raman spectroscopy is unable to obtain locally heterogeneous information, TERS can uncover them with nanometer precision. Smithe
For optoelectronic devices the physics of semiconductor–metal interfaces is critical, since they are at the core of charge separation and collection. Jariwala
4.3 Semiconducting to metallic phase transition
The stable crystal structure of TMDCs is the 2
Milekhin
4.4 Probing edge related properties
TMDCs have two distinct edges in the form of armchair and zigzag, which possess different electronic properties. For example, the zigzag edge in MoS2 is metallic, whereas the armchair is semiconducting [85]. Therefore, quantitative measurements or characterization of these edges are important for effective edge engineering, especially for edge contacts for electronic device applications. Huang
A broad Raman feature around 220 cm−1 was observed along the edges (full of dangling bonds) and absent on the wrinkles and main body (Figure 9a). The peak was assigned to the defect (dangling bonds) induced acoustic
Another important observation of this study is the edge dependent local Raman sensitivity. The frequency position of the out-of-plane
5. Conclusion and outlook
TERS has developed into a versatile characterization technique for a wide range of materials. In recent years we have witnessed chemical mapping of single molecules with a spatial resolution of the bond length using TERS. For 2D semiconductors, an excellent SR of ∼2 nm in TERS helped us to understand local heterogeneous behavior of monolayer MoS2 and demonstrate its tremendous capabilities. In this chapter, we explained the basic principal of TERS and described the surface selection rules in regards to the 2D semiconductors important for understanding the near-field Raman spectra. We also reviewed the recent progress of this technique in the 2D semiconductor field. The potential of TERS certainly guarantees new breakthroughs in the 2D semiconductor field in the near future. One of the recent direction of 2D semiconductors is to create heterostacks like Lego, which promises exotic physics due to the creation of moiré superlattices. The size of a moiré unit cell varies from nm to few 10s nm depending on the lattice mismatch and rotation angle. The moiré superlattice forces the constituent monolayers into phonon renormalization, also known as moiré phonons. TERS can be used to probe these moiré phonons, thus resolving the moiré supercell critical for understanding the moiré physics. Besides, there is a lot of information yet to be resolved on how local heterogeneities control the optical response such as strain induced exciton funneling, defects induced excitonic response, or single photon emitters
Acknowledgments
This work was supported by the German research council funded project DFG-ZA 146/44-1.
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