Atomic-Scale Variations of the Mechanical Response of 2D Materials Detected by Noncontact Atomic Force Microscopy

Abstract

We show that noncontact atomic force microscopy (AFM) is sensitive to the local stiffness in the atomicscale limit on weakly coupled 2D materials, as graphene on metals. Our large amplitude AFM topography and dissipation images under ultrahigh vacuum and low temperature resolve the atomic and moiré patterns in graphene on Pt(111), despite its extremely low geometric corrugation. The imaging mechanisms are identified with a multiscale model based on density-functional theory calculations, where the energy cost of global and local deformations of graphene competes with short-range chemical and long-range van der Waals interactions. Atomic contrast is related with short-range tip-sample interactions, while the dissipation can be understood in terms of global deformations in the weakly coupled graphene layer. Remarkably, the observed moiré modulation is linked with the subtle variations of the local interplanar graphene-substrate interaction, opening a new route to explore the local mechanical properties of 2D materials at the atomic scale. DOI: 10.1103/PhysRevLett.116.245502 Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) are tools of choice for characterizing the unique mechanical and electronic properties of graphene (G) and other 2D materials. Dynamic AFM [1] in the frequency modulation (FM) mode [2] has resolved the true geometric structure of a broad range of materials [3][4][5]. FM AFM experiments on carbon-based materials [6][7][8][9][10][11][12][13] show atomic contrast in Δf images and, depending on the setup, in the dissipation channel. While the origin of the dissipation is not well understood, the Δf contrast has been linked with the nature of the tip-sample interaction [14].G properties can be efficiently tuned by the interaction with metals [15,16]. The interaction strength varies widely from the strong coupling with Rh [17,18] and Ru [19] to the weak limit (Ir [20], Pt [21]), where G retains its unique electronic properties [22]. The different lattice parameters of G and the metal underneath are accommodated through the formation of commensurate structures known as moiré patterns, where C atoms become inequivalent due to their different bonding configuration with the metal. The resulting "true" topographic corrugation of G-the difference in height among the topmost and the bottom C atom-varies widely, even in the weakly interacting cases, where it ranges from ≈50 pm on Ir [23,24] to practically flat (≤ 3 pm) on Pt [21].While STM can easily resolve these moiré patterns, even in the G=Pt case [21,25], AFM experiments have only been reported in highly corrugated cases as Ru [26], Rh [27], and Ir [12,28]. Focusing on the most challenging case, G=Ir, experiments with a Kolibri sensor using a W tip clearly resolved the moiré in constant height (CH) AFM images [28]. Measurements with a tuning fork using both inert (CO-terminated) and reactive (Ir-terminated) tips [12] were able to identify the atoms with both tips at any tip-sample distance. This atomic-scale resolution allowed the ...

Atomic force microscopy contrast with CO functionalized tips in hydrogen-bonded molecular layers: Does the real tip charge distribution play a role?

“…The approximated DFT energy is supplemented using a semiempirical vdW approach. Specifically, we use Grimme's DFT-D3 theory [19], which for AFM calculations complements well the KS energy [20]. Lastly, the potential V tilt (θ,ϕ) accounts for the penalty of a probe tilt.…”

“…The SAM induces a small rotation of the preferred adsorption sites of the isolated triazines on the G: the C atoms of the triazine on top of the G's C atoms. Triazine has been experimentally synthesized on a G/Pt(111) substrate [23], which is a weakly interacting G-metal system with a tiny corrugation [20].…”

Guide to optical spectroscopy of layered semiconductors

“…This will be important for understanding the origin of localized emitters and how optical properties depend on moiré periodicity. Nanometre-scale moiré superlattices can be imaged by using near-field spectroscopy techniques, SEM, atomic-resolution transmission electron microscopy, atomic force microscopy or scanning tunnelling spectroscopy 13,14,200,201 . One possible way to increase spatial resolution in optical spectroscopy below the diffraction limit is to perform tip-enhanced spectroscopy, as recently demonstrated 202 .…”

Nanomechanical Properties of Epitaxial Silicene Revealed by Noncontact Atomic Force Microscopy

“…The resulting energy loss at each cycle is compensated in such a way that the oscillation amplitude is kept constant by a feedback loop. The variation of excitation energy output from the feedback loop, with respect to that of the tip far from the surface, called the energy dissipation, is mapped and gives the dissipation image . In the atomic‐resolution nc‐AFM measurement, topography, and dissipation images are simultaneously recorded which allows for analyzing the interaction at atomic scale and evaluating the mechanical response of surface atoms.…”

“…The interaction of the tip with mechanically flexible epitaxial silicene on ZrB 2 can therefore be naively described as the sum of the interactions with three different atomic layers: the on‐top atoms, the intermediate layer and the ZrB 2 (0001) surface. This situation can be compared to the interaction of a tip with a flexible sheet of graphene on Pt(111) investigated by nc‐AFM and theoretical calculations which revealed the deformation of the graphene sheet …”