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Yao Zhang, Yang Zhang, Zhen-chao Dong. Scanning Raman Picoscopy: Ångström-Resolved Tip-Enhanced Raman Spectromicroscopy†[J]. Chinese Journal of Chemical Physics , 2021, 34(1): 1-14. doi: 10.1063/1674-0068/cjcp2102027
Citation: Yao Zhang, Yang Zhang, Zhen-chao Dong. Scanning Raman Picoscopy: Ångström-Resolved Tip-Enhanced Raman Spectromicroscopy[J]. Chinese Journal of Chemical Physics , 2021, 34(1): 1-14. doi: 10.1063/1674-0068/cjcp2102027

Scanning Raman Picoscopy: Ångström-Resolved Tip-Enhanced Raman Spectromicroscopy

doi: 10.1063/1674-0068/cjcp2102027
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  • Corresponding author: Zhen-chao Dong, E-mail: zcdong@ustc.edu.cn
  • Part of special topic of "the New Advanced Experimental Techniques on Chemical Physics".
  • Received Date: 2021-02-03
  • Accepted Date: 2021-02-22
  • Publish Date: 2021-02-27
  • In this review, we present a brief overview on the recent advances in Ångström-resolved tip-enhanced Raman spectromicroscopy. We first introduce the theoretical understanding of the confinement of light at the atomistic scale, and explain how the Raman scattering from a single molecule happens under the "illumination" of such an atomically confined light. Then we describe the latest developments on Ångström-resolved tip-enhanced Raman spectromicroscopy, particularly on a new methodology called "scanning Raman picoscopy" for visually constructing the chemical structure of a single molecule in real space. Finally, we give a perspective of this technique in various applications where identifying the chemical structures of materials at the chemical bond level is required.
  • Part of special topic of "the New Advanced Experimental Techniques on Chemical Physics".
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Scanning Raman Picoscopy: Ångström-Resolved Tip-Enhanced Raman Spectromicroscopy

doi: 10.1063/1674-0068/cjcp2102027

Abstract: In this review, we present a brief overview on the recent advances in Ångström-resolved tip-enhanced Raman spectromicroscopy. We first introduce the theoretical understanding of the confinement of light at the atomistic scale, and explain how the Raman scattering from a single molecule happens under the "illumination" of such an atomically confined light. Then we describe the latest developments on Ångström-resolved tip-enhanced Raman spectromicroscopy, particularly on a new methodology called "scanning Raman picoscopy" for visually constructing the chemical structure of a single molecule in real space. Finally, we give a perspective of this technique in various applications where identifying the chemical structures of materials at the chemical bond level is required.

Part of special topic of "the New Advanced Experimental Techniques on Chemical Physics".
Yao Zhang, Yang Zhang, Zhen-chao Dong. Scanning Raman Picoscopy: Ångström-Resolved Tip-Enhanced Raman Spectromicroscopy†[J]. Chinese Journal of Chemical Physics , 2021, 34(1): 1-14. doi: 10.1063/1674-0068/cjcp2102027
Citation: Yao Zhang, Yang Zhang, Zhen-chao Dong. Scanning Raman Picoscopy: Ångström-Resolved Tip-Enhanced Raman Spectromicroscopy[J]. Chinese Journal of Chemical Physics , 2021, 34(1): 1-14. doi: 10.1063/1674-0068/cjcp2102027
  • Since tip-enhanced Raman spectroscopy (TERS) was first experimentally demonstrated in 2000 [1-4], it has shown powerful capabilities to visualize individual molecules with chemical identification [5-11]. This technique can exhibit both sensitive chemical specificity and high spatial resolution at the sub-nanometer scale, especially by combining with scanning tunneling microscopy (STM) or atomic force microscopy (AFM) operated at ultrahigh-vacuum and low-temperature conditions [12-19]. Thanks to the strong spatial confinement and enhancement of local electromagnetic fields at the apex of a metallic probing tip [20-22], Ångström-resolved Raman images of a single molecule have also been achieved [23-26], offering a promise to optically determine the chemical structure of a single molecule in real space.

    The determination of the chemical structure of a molecule is an important issue in chemistry, catalysis, material science, and biology. Different spectroscopic tools, such as nuclear magnetic resonance [27], electronic and vibrational spectroscopies [28-30], have been routinely employed to characterize the molecular structures. The rich spectroscopic data and chemical intuition help to identify the basic chemical groups or specific chemical bonds in a molecule. However, the lack of spatial information has made it very difficult to firmly determine the placement and connectivity of the chemical groups from the spectroscopic data alone. STM and AFM have the remarkable ability to visualize the molecular skeleton [31-36], but usually lack sufficient chemical information for precise chemical structure determination. Such deficiencies can in principle be overcome by combining with the sub-nanometer resolved TERS. In this review, instead of presenting a comprehensive overview of TERS, we focus on the recent advances in Ångström-resolved tip-enhanced Raman spectromicroscopy. We first give a brief introduction of the theoretical treatment on the confinement of light at the atomistic scale. Then the Raman scattering of a single molecule under the "illumination" of such an atomically confined light is discussed, which is found to be extremely sensitive to the spatial distribution of local fields. Finally, we briefly overview recent developments on Ångström-resolved tip-enhanced Raman spectromicroscopy, especially on the new scanning Raman picoscopy (SRP) methodology for visually constructing the chemical structure in real space. The review is concluded with a perspective of the SRP technique in various applications.

  • To achieve ultrahigh spatial resolution in optical imaging, one of the desirable approaches is to have highly confined electromagnetic fields. In the presence of nanostructures, the excitation of the localized surface plasmons (LSPs) would result in the confinement and enhancement of the local electromagnetic fields near the metal surface, and there is no fundamental limit preventing such confinement down to atomic scales [37]. In order to achieve such confinement, the atomic-scale control of the nanostructure morphologies and gap structures are required. For the classical methods that are adopted to obtain the local fields based on the solution of Maxwell's equations [37-45], the local field properties are usually determined by the smooth boundaries of a nanoscale morphology of the nanostructure (tip or tip-substrate cavity) with a more or less sophisticated description of the roughness. FIG. 1 (a) and (b) show typical results of the local field enhancement in nanocavities based on the classical or quantum simulations, respectively. As shown in FIG. 1(a), for a nanocavity defined by the nanosphere dimer, the boundaries are described by two smooth spherical surfaces and the plasmonic properties can be simulated by the plasmon hybridization (PH) and finite-difference time-domain (FDTD) methods [46]. The plasmonic field is found to be strongly enhanced and highly localized within the gap due to the strong interaction and hybridization between two nanospheres. Even in the quantum description of the optical response of nanocavities, the electronic density at the metal-vacuum interface is often considered to be smoothly distributed, as shown in FIG. 1(b), treating the electron gas within the framework of a jellium model. Such a treatment is valid for obtaining the optical response of metallic nanoresonators through using the time-dependent density functional theory (TDDFT) [47-51]. FIG. 1(b) shows the snapshot of induced current density, induced charge density and the field intensity of the nanosphere dimer under the linear response regime from the quantum model [51]. A large charge density is induced at the surfaces between the nanogaps, resulting in the strong local electric field enhancement.

    Figure 1.  Localization of electric fields in the plasmonic nanogap using different classical and quantum simulation methods. (a) Local field distributions simulated by PH and FDTD methods, respectively. Adapted with permission from Ref.[46] © Springer. (b) and (c) Simulated local electric fields in the gap with an "atomistic" protrusion by FDTD (adapted with permission from Ref.[22] © American Association for the Advancement of Science) and FEM (adapted with permission from Ref.[58] © The Royal Society of Chemistry), respectively. (d) The snapshot of induced current density, induced charge density and the field intensity of the nanosphere dimer under the jellium model (adapted with permission from Ref.[51] © American Chemical Society). (e) and (f) Full quantum atomistic calculations of the local electric field of a dimer based on TDDFT (adapted with permission from Ref.[21] © American Chemical Society) and semiempirical tight-binding model (adapted with permission from Ref.[59] © Springer), respectively.

    Beyond these continuous descriptions of smooth boundaries for the plasmonic nanostructures, atomistic models that refer the optical response of matter to its atomic constituents can reveal the role of subnanometric features in nanocavities and their actions onto the nearby molecules through the highly confined local field [52-55]. Recent studies have shown that the spatial resolution of tip-enhanced spectroscopy has now reached the subnanometer and even Ångström level [23, 24, 56, 57], which relies on the exquisite control of the localization of optical near-fields to a few atoms at the apex of a metallic scanning tip. In classical simulations, such atomistic features can be mimicked by introducing an atomic-scale protrusion at the apex of the tip. FIG. 1 (c) and (d) show the simulated local electric field in the gap composed of the "atomistic" tip and the substrate using FDTD [22] and finite element method (FEM) [58], respectively. The so-called "picocavity" is produced in such a plasmonic gap with atomic-scale protrusions that localize the induced near fields to the atomic scale, thus providing high spatial resolution for the optical property studies at the submolecular or even Ångström scale. FIG. 1(e) shows a full quantum atomistic calculation of the plasmonic response of a metallic cavity based on TDDFT, which demonstrates the localization of plasmonic local fields below 1 nm$ ^3 $ at atomic-scale vertices and edges [21]. Note that similar results can be also obtained from the semiempirical tight-binding model [59], as shown in FIG. 1(f).

    Within the classical theory, the enhancement and confinement of the local field near the metallic tip is generally evaluated by assuming the tip as a nanosphere with a radius $ R $, as shown in FIG. 2(a). Upon the illumination of an incident light ($ \textbf{E}_{ \rm{inc}} $), the local electric field ($ \textbf{E}_{ \rm{loc}} $) at the distance $ \textbf{r} $ near the spherical surface can be obtained from the following equation [17]:

    Figure 2.  (a, b) Schematic of the geometric structures and local field distributions of an individual metal tip as well as its analogue as a single sphere (a) and a metal tip-substrate structure as well as its analogue as a sphere dimer (b). Panels (a) and (b) are adapted with permission from Ref.[17] © The Royal Society of Chemistry. (c) Sketch showing one apex out of an icosahedral atomistic nanoparticle (circles represent atoms). (d) Sketch of an atomistic protrusion on a spherical nanoparticle that produces an atomic lightning rod effect enhancing the background plasmonic field given by the spherical nanoparticle. (e) Comparisons between the quantum, classical and analytical simulation results of local electric fields, exhibiting a similar feature in both field intensities and spatial distributions. Panels (c)-(e) are adapted with permission from Ref.[60] © American Chemical Society.

    where $ \varepsilon_ \rm{m} $ is the permittivity of the metal sphere. Therefore, the local field enhancement right beneath the tip apex with a separation distance $ d $ can be obtained as

    The local field gets significantly enhanced approaching the surface ($ d $$ \rightarrow $0), and reaches a maximum enhancement at the resonance condition ($ \varepsilon_m $+2 = 0). In other words, the incident light should be resonant with the local plasmonic mode of the nanostructure for achieving maximum enhancement. The confinement of such a local field ($ w $) can be obtained by examining the field distribution laterally shifted from the center position [17], giving a value of the full width at the half maximum (FWHM) as $ w $$ \propto $$ R $+$ d $. Therefore, in order to get a more localized field, the curvature radius of the tip should be as small as possible, that is, a very sharp tip (that could eventually imply even an atomic-scale sharpness) is required to achieve ultrahigh spatial resolution. Nevertheless, the enhancement might decrease as the tip becomes sharper, which can also be demonstrated from Eq.(2). If the separation distance $ d $ is fixed, the smaller the radius of the tip, the smaller the local field enhancement. Conversely, the bulky tip shaft is also very important in terms of the local field enhancement.

    By further introducing a substrate to form a nano-gap and the resultant gap-mode plasmon, which can be regarded as a dimer system composed by a nanosphere and its image (FIG. 2(b)), the local electric field could be further enhanced and confined by tuning the gap distance through the following equation [17]

    Thus, as the two spheres get closer, the local field intensity would increase dramatically. In this case, the confinement of the local field could be also obtained via $ w $$ \propto $$ \sqrt {Rd} $, which means that even when the tip radius is not sharp enough (which is usually the case in experiments), both the local field enhancement and the lateral confinement can still be improved by decreasing the gap distance.

    However, once the gap distance enters the Ångström scale, the size of the nanocavity becomes comparable to the atomic radii of metals. In this situation, the real atomistic structures of the nanostructures should be considered during the calculation of the optical response of the nanocavity. In analogy to the classical macroscopic situation of the lightning rod effect in which the field enhancement follows a strong potential gradient due to the curvature of a metallic interface, an "atomic-scale lightning rod effect" is produced for the local field in the proximity of an atomistic protrusion [60]. Such an effect is responsible for the additional localization at the tip apex, as shown in FIG. 2 (c) and (d). For such tip structures, with an atomistic protrusion attached to a tip shaft, the fields are first localized and enhanced near the surface, producing a background plasmonic field associated with the tip shaft (which is strongly enhanced especially at the resonance condition); the lightning rod effect associated with the atomistic protrusion would produce an additional enhancement over this background, resulting in an atomic-scale "hot spot". By comparing with the quantum calculations using the TDDFT method [21, 60], it can be found that the main features of the field localization and enhancement in atomistic protrusions can be correctly addressed by the classical and analytical simulations through the consideration of the sharp curvature of the atomistic profile (namely without considering the details of atomistic distributions), as demonstrated by the similarities of the near field distributions in FIG. 2(e). Therefore, the spatial extension of the enhanced local field can go down to atomic scale for an atomistic protrusion, which is responsible for the high spatial resolution of TERS mapping images [21, 58, 60, 61].

  • For a molecule under the illumination of an incident light, the Raman scattering process can be described as the polarization of molecular electronic densities that can be influenced by different molecular vibrations, resulting in the inelastic scattering of the incident light. Usually, the molecule is regarded as a point dipole ($ \textbf{p}_{ \rm{mol}} $) and the optical response of the molecule can be described by the polarizability ($ \alpha_{ \rm{mol}} $) as

    where $ \rm{E}_{ \rm{loc}} $($ \textbf{r}_{ \rm{mol}} $) is the local electric field at the molecular position $ \textbf{r}_{ \rm{mol}} $. As shown in the left panel of FIG. 3(a), the local field is often assumed to be homogeneous over the whole molecule and it is thus reasonable to adopt the point-dipole approximation. However, as we have described above, the local electric field can be confined at the atomic scale with a size comparable to or even smaller than the size of a molecule (right panel in FIG. 3(a)). In this case, the point-dipole approximation would not be valid because the field distribution is not homogeneous over the molecule, and different atoms inside a molecule may experience different polarizations induced by the position-dependent local field. Generally, the polarized molecular dipole moment induced by a local electric field can be expressed as [24, 25, 61, 62]:

    Figure 3.  (a) Schematic of different situations of a single molecule from the homogeneous field to the atomically localized field. (b) Variations of local electronic densities of a single C-H bond during the stretching vibrations. (c) Variation of the local electronic density of a single MgP molecule corresponding to the anti-symmetric (top) and symmetric (bottom) stretching vibrations of two C-H bonds in one pyrrole ring.

    where $ \hbar $ is Planck's constant, $ {\Psi _{i\left( {r, f} \right)}} $ is the molecular wavefunction of the initial (intermediate, final) state, $ {\omega _{ri\left( {rf} \right)}} $ is the transition energy between state $ r $ and state $ i(f) $, and $ \omega $ is the frequency of electric field. For a homogeneous field, we can define the molecular polarizability $ {\alpha _{\rho \sigma }} $ which is generally defined as

    where $ \rho $($ \sigma $) refers to the $ x $-, $ y $- or $ z $-component of transition dipole moment operators. In order to consider the contributions from molecular vibrations, we can expand the integral term in the normal coordinates under the Born-Oppenheimer approximation as

    where $ \left| {{\psi _{i\left( r \right)}}} \right\rangle $ is the molecular electronic wavefunction of the initial (intermediate) state, $ \left| {{v_i}\left( {{{v'_r}}} \right)} \right\rangle $ is the molecular vibrational wavefunction of the initial (intermediate) state, and $ Q_k $ is the normal coordinate of the $ k $-th vibrational mode. By applying the coordinate transformation to the second term in above equation from the normal-mode coordinates to the atomic Cartesian coordinates, we have

    By substituting Eq.(7) and Eq.(8) into Eq.(5), and assuming that only the local electronic wavefunctions near the vibrating atoms play a dominant role, the polarized molecular dipole moment corresponding to the $ k $-th vibrational mode can be attributed to the contributions from each atom in the molecule [24, 61]:

    where the effective local electric field within this atomic "nearby" region can be regarded as the field at the corresponding atomic position $ {{\bf{E}}^{{\rm{loc}}}}\left( {{{\bf{r}}_n}} \right) $. Here we have defined $ \textbf{p}_{n, k} $ as the atomistic Raman dipole moment of the $ n $-th atom in the molecule corresponding to the $ k $-th vibrational mode induced by the local electric field. The emission of a collection of such polarized molecular dipoles would be further scattered by the nanocavity acting as a nano-antenna through the Green's function $ {{\bf{\stackrel{\leftrightarrow}{\mathbf{G}}_{} }}_k}\left( {{{\bf{r}}_\infty }, {{\bf{r}}_n}} \right) $, resulting in the Raman scattering cross-section as

    Therefore, within the above framework, only the contributions from the vibrating atoms within the local field region need to be considered because of their relatively large polarized atomic dipole moments. In other words, only the atomic vibrations within the local field region would contribute dominantly to the Raman signals from a single molecule.

    Taking the C-H stretching vibration within a pyrrole ring as an example, FIG. 3 (b) and (c) show how the atomic vibrations and related phases of neighboring atoms and chemical bonds within the local field would influence the overall Raman signals. As shown in FIG. 3(b), for a single C-H bond, the compression of the bond length would result in the increase of local electronic density $ \rho $, while the stretching would result in a decrease in $ \rho $. Considering the relation between the dipole moment p and the electronic density $ \rho $, i.e., $ {\bf{p}} $ = -$ { \int} {\rho {\bf{r}} \rm{d}{\bf{r}}} $, the polarizability derivatives can be written as

    Therefore, the real-space perturbation of molecular vibrations (i.e., mode amplitudes and phases) on the electronic densities can provide insights into the phase relations of different vibrations of neighboring bonds. In this sense, for the two C-H bonds corresponding to the C-H stretching vibration within one pyrrole ring, as shown in FIG. 3(c), the mutual interaction between them will result in two vibrational modes: one corresponds to the symmetric vibration of two C-H bonds stretching in-phase, and the other corresponds to the anti-symmetric vibration of two C-H bonds stretching out-of-phase. If the tip is located above one single C-H bond, only the vibration of a single bond can be observed. However, if the tip is located in the middle between two C-H bonds, different phases for the symmetric and anti-symmetric vibrations would result in different interference effects between these two C-H bonds, as demonstrated by the electronic density derivatives in FIG. 3(c) [24]. For the symmetric mode, the contributions from two stretching C-H bonds will sum up and result in a constructive interference phenomenon, while for the anti-symmetric mode the contributions from two stretching C-H bonds will cancel out each other, resulting in a destructive interference phenomenon. In other words, the in-phase local vibrations carry the same sign in polarization, while the out-of-phase local vibrations carry the opposite signs, giving the null integral value under the sampling window of the local field. Therefore, by confining the electric field at the atomic scale, the Raman signals from the single chemical bonds and even the phase relations between the vibrations of neighboring bonds can be directly visualized by TERS mapping in real space.

  • The high-spatial-resolution TERS technique not only allows to distinguish different molecules and surface structures at single-molecule level, but also enables the exploration of internal structures within a single molecule. In 2008, Steidtner et al. first reported the TERS-based microscopy on single dye molecules with 15 nm resolution [8], though the spatial details in mapping images are relatively limited (FIG. 4(a)). In 2013, Zhang et al. pushed the spatial resolution of TERS mapping down to sub-nanometer scale ($ \sim $0.5 nm) by adopting a double-resonance scheme and under ultrahigh vacuum and liquid nitrogen temperature, and optically resolved the intramolecular structure of a single porphyrin (H$ _2 $TBPP) molecule on Ag(111) (FIG. 4(b)) [12]. Since then, great efforts have been devoted to improving the spatial resolution further into Ångström level for identifying the chemical groups and even single chemical bonds in real space. In 2019, Lee et al. reported the spatial mapping for a single Co(Ⅱ)-tetraphenyl porphyrin (CoTPP) molecule on Cu(100) with a spatial resolution down to $ \sim $1.67 Å [23]. As shown in FIG. 5(a), each Raman peak shows its own characteristic image with rich details, which can be assigned to different vibrational modes. Jaculbia et al. also reported the single-molecule resonance Raman effect of an isolated copper naphthalocyanine (CuNc) molecule on a three-monolayer-thick NaCl film supported by the Ag(111) substrate [63]. The Raman images present different patterns depending on the symmetries of the vibrational modes (FIG. 5(b)). Such high-spatial-resolution TERS has also been applied to different systems for identifying chemical species and local structures at the single-molecule level, including distinguishing adjacent different molecules [13, 16, 64-66], visualizing carbon nanotubes [67, 68] and single DNA/RNA chains [69-71], and specifying catalytic sites on surfaces [72, 73] and defects in two-dimensional materials [74, 75].

    Figure 4.  (a) TERS mapping on single dye molecules with 15 nm spatial resolution. Adapted with permission from Ref.[8] © The American Physical Society. (b) Chemical mapping of a single H$ _2 $TBPP molecule on Ag(111). Adapted with permission from Ref.[12] © Springer Nature.

    Figure 5.  (a) TERS mapping images for typical vibrational normal modes of CoTPP on Cu(100). Adapted with permission from Ref.[23] © Springer Nature. (b) Resonance Raman mapping images of CuNc on NaCl/Ag(111). Adapted with permission from Ref.[63] © Springer Nature.

  • The recent advances in TERS imaging down to sub-nanometer or even Ångström scale, as described above, have stimulated extensive interests on the theoretical understanding of the origin of ultrahigh spatial resolution [17, 22, 25, 26, 76-79]. One of the most possible explanations is believed to associate with the presence of an atomistic protrusion at the tip apex, forming a picocavity to highly confine the local electric field within a sub-nanometer scale [21, 22, 60]. This would require an exquisite control over the tip structure and stability (the tip quality is absolutely crucial for TERS experiments!). Therefore, before introducing the new SRP methodology for structural reconstruction, we would like to highlight the importance of ultrahigh-vacuum and low-temperature conditions for fabricating and maintaining a stable plasmonic picocavity (more specifically, the atomistic protrusion at the tip apex) so that Ångström-resolved TERS imaging can be achieved, among other advantages such as contamination-free single-molecule sample preparation and good thermal stability [80].

    Because the local field can be confined into a picocavity and the local Raman response can be attributed to the contributions from the vibrating atoms within a molecule, the full vibrational imaging of a molecule in real space has also been demonstrated for Mg-porphine (MgP), with a spatial resolution of $ \sim $1.5 Å and a chemical sensitivity down to a single chemical bond (FIG. 6) [24]. Since full vibrational imaging of a molecule can reveal molecular structural details associated with distinct characteristics of each vibrational mode in real space, the chemical structure of a single molecule can be reconstructed through a simple Lego-like building process, as demonstrated for a model MgP molecule through the so-called SRP technique [24].

    Figure 6.  Schematic of scanning Raman picoscopy of MgP/Ag(100) system, typical Raman spectra at representative positions (lobe, gap and center) and the spatial mapping images for the labelled Raman peaks. The line profile of Raman signal intensities corresponding to the dashed line is also shown, exhibiting a lateral spatial resolution down to $ \sim $1.5 Å. Adapted with permission from Ref.[24] © Oxford University Press on behalf of China Science Publishing & Media Ltd.

    FIG. 7 shows the assembling process from the identification of different chemical bonds/groups to the construction of the "unknown" MgP molecular chemical structure (pretending that only the types of atomic elements are known), by exploiting the underlying interference effect in various vibrational imaging and Raman fingerprint database [24]. For example, for the highly localized C-H stretching vibrations in the range of 2800-3400 cm$ ^{-1} $, two-peak features at 3072 and 3092 cm$ ^{-1} $ can be distinguished and identified as the sp$ ^2 $ C-H stretching modes. The corresponding spatial mapping images show distinct patterns: the one at 3072 cm$ ^{-1} $ is composed of "eight bright dot", while the other at 3092 cm$ ^{-1} $ consists of "four lobes" (upper-left panel in FIG. 7). Considering the interference effect discussed in FIG. 3, the well-resolved eight dots mark the positions of eight C-H bonds stemming from the out-of-phase destructive interference associated with anti-symmetric vibrations, while the four lobes are generated from the in-phase constructive interference between two neighboring C-H bonds associated with symmetric vibration. As a result, the intensities in-between the neighboring dots would be the weakest for the anti-symmetric mode, while for the symmetric mode the brightest spot would show up in the lobe close to the center of two connected C-H bonds. Thus, the first Lego piece of the molecule can be determined to be H-C = C-H groups.

    Figure 7.  Reconstructing full molecular structure by assembling representative units of chemical groups marked with different colors: pyrrole rings in red, pyrrole C-H bonds in yellow, bridging C-H bonds in green, and central Mg atom in blue. Adapted with permission from Ref.[24] © Oxford University Press on behalf of China Science Publishing & Media Ltd.

    Following similar procedures, the spectral region in 1300-1700 cm$ ^{-1} $ can be correlated with C = C stretching vibrations, and a conjugated five-membered pyrrole ring structure can be determined for each lobe in the mapping images corresponding to the Raman peaks in this region (upper-right panel of FIG. 7). The connecting piece between the neighboring pyrrole rings can be determined to be C-H bonds (lower-left panel of FIG. 7) through the related out-of-plane bending vibrations, and the central atom can be identified as the Mg metal atom from the images of low-frequency vibrations (lower-right panel of FIG. 7). With all these pieces identified, the chemical structure of the target MgP molecule is fully determined in real space (middle of FIG. 7). By overlaying the representative individual vibrational mapping images with different colors to generate a merged image, the spatial arrangement of individual chemical groups can be clearly demonstrated, in nice coincidance with the artistic view of the MgP molecule [24].

    The SRP technique can be also applied to other molecular systems. For the H$ _2 $TBPP molecule adsorbed on the Ag(100) surface, the orientation of N-H bonds around the porphyrin core can be directly identified through spatial TERS mapping [81]. For the [12]CPP molecule, the SRP technique can be used to specify different adsorption configurations and structural deformation on metal substrates with different crystallographic orientations [82]. Especially for the [12]CPP on Ag(110), the molecular shape is found to be severely deformed, and the spatial mapping images can provide a direct and panoramic view on the local structural influences caused by different tilting of the benzene units in real space [82], as shown in FIG. 8.

    Figure 8.  TERS mapping of a single [12]CPP molecule on Ag(110) for representative vibrational modes. The schematics of vibrations for the involved benzene units for each mode are shown on the left, and the molecular structures highlighting the atoms involved for each mode are shown on the right in pink color. Adapted with permission from Ref.[82] © American Institute of Physics.

  • In this review, we have overviewed the basic concepts and recent advances of the Ångström-resolved tip-enhanced Raman spectromicroscopy. We first introduced theoretical background of the confinement of light at the atomistic scale and the Raman scattering from a single molecule under the "illumination" of such an atomically confined light. Then we briefly described the latest developments on Ångström-resolved tip-enhanced Raman spectromicroscopy, and focused on a new SRP methodology for the molecular structural reconstruction to show how such ultrahigh-resolution spectromicroscopic vibrational images can be used to visually assemble the chemical structure of a single molecule through a simple Lego-like building process. The SRP protocol can be widely applied for identifying the chemical structure of different materials at the level of chemical bonds, as illustrated in FIG. 9. First, this technique can be used to study the upstanding [83] or 3D adsorbed molecules due to its vertical resolving ability and even determine the stereostructure through a tomographic process; Second, it is also able to analyze the chemical structures (including defects [74, 75], surface reconstructions [84], etc.) of surface and 2D materials (such as graphene and transition-metal dichalcogenides); Third, the Lego-like building process can be also generalized with the aid of artificial intelligence (AI) including imaging recognition and machine learning, and becomes even more powerful by combining with non-contact AFM [85] and inelastic tunneling probe techniques; Fourth, further combination with ultrafast spectroscopic techniques may extend the applications of SRP technique from static studies to dynamics studies with high spatial-temporal resolution, helping to reveal the mechanism of physical processes such as surface reactions and photo-catalysis. All these applications would make the SRP technique as a versatile tool to probe and identify chemical structures in various systems at the single-chemical-bond level.

    Figure 9.  Exploitation of SRP technique for different applications: determination of stereostructure of adsorbed single molecules, analysis of chemical structures of surface and 2D materials, AI reconstruction of the chemical structures in combination with image recognition and machine learning, and the specification of structural changes from reactants to products in surface reactions.

  • This work was supported by the National Key R&D Program of China (No.2016YFA0200600), the National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences (No.XDB36000000), and the Anhui Initiative in Quantum Information Technologies.

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