AVS 66 Session 2D+AS+MI+NS-WeM: 2D Materials Characterization by Scanning Probe Microscopy and Spectroscopy

In this contribution we report a careful investigation of the topographic, electronic, and opto-electronic properties of electronically decoupled C60 nanocrystals by means of Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), STM induced Light Emission (STM-LE), and Density Functional Theory (DFT), at the single molecule level. We have observed that depending on the tunneling parameters, we are able to reversibly switch from a broad plasmonic spectrum to a series of very sharp molecular resonances of 10meV width. Based on our STS data and DFT calculations we have reached a model to explain the molecular light emission of the C60 nanocrystals. We demonstrate that the emission arises from the recombination of a doublet exciton of negatively charged molecules (anions). Our results will contribute to the general understanding of the physical mechanisms involved in the light emission of organic molecules. They may also be of importance for applications such as single photon emitters or quantum computing, since future devices may need to be able to switch chromaticity in a controlled fashion.

In a second part, we will address experimental and theoretical studies of monolayer topological superconductivity and chiral Majorana edge modes in model-type 2D magnetic Fe islands on elemental superconducting Re [7]. In particular, we demonstrate that interface engineering by an atomically thin oxide layer is crucial for driving the studied hybrid system into a topologically non-trivial state as confirmed by theoretical calculations of the topological invariant, the Chern number.

This work is supported by the EU via the ERC Advanced Grant No. 786020 “ADMIRE”.

[1] A. Kamlapure et al., Phys. Rev. B 95, 104509 (2017).

[2] S. Manna et al., Nature Commun. 8, 14074 (2017).

[3] S. Meckler et al., Rev. Sci. Instrum. 80, 023708 (2009).

[4] T. Hänke et al., Nature Commun. 8, 13939 (2017).

[5] J. Warmuth et al., NPG Quantum Materials 3, 21 (2018).

[6] U. R. Singh et al., Phys. Rev. B 97, 144513 (2018).

[7] A. Palacio-Morales, E. Mascot, S. Cocklin, H. Kim, S. Rachel, D. K. Morr, and R. Wiesendanger, arXiv:1809.04503 (2018).

Two-dimensional (2D) boron, known as borophene, has recently been experimentally realized^1,2 revealing a number of polymorphic structures. A common characteristic of those borophene polymorphs is the presence of hollow-hexagons (HH) in an otherwise triangular lattice. The vast number of possible HH lattices underlies the polymorphic nature of borophene. Superlattices of HHs could be further realized when borophene phases periodically intermix in the form of line defects³. While scanning tunneling microscopy (STM) of borophene with conventional metal probes has revealed key features of borophene, significant ambiguity of the exact atomic lattice structure exists due to the convolution of electronic and structural details. With CO-functionalized atomic force microscopy, we unambiguously resolve the HH lattice and reveal features corresponding to B-B covalent bonds⁴ that are supported by numerical simulations. We further show that CO-functionalized STM is an equivalent but more accessible technique for HH imaging, allowing us to assemble a growth phase diagram of borophene involving non-rotated, 30°-rotated and randomly rotated borophene phases on Ag(111), confirming the v_1/5 and v_1/6 models as the unifying structures for all observed phases. In particular, a transition from rotationally commensurate to incommensurate phases is observed at high growth temperatures, corroborating the chemically discrete nature of borophene.

*Current affiliation of Xiaolong Liu: Kavli Postdoc Fellow, LASSP, Cornell University

1. A. J. Mannix, X.-F. Zhou, B. Kiraly, J. D. Wood, D. Alducin, B. D. Myers, X. Liu, B. L. Fisher, U. Santiago, J. R. Guest, M. J. Yacaman, A. Ponce, A. R. Oganov, M. C. Hersam, N. P. Guisinger, Science 350, 1513–1516 (2015).

2. B. Feng, J. Zhang, Q. Zhong, W. Li, S. Li, H. Li, P. Cheng, S. Meng, L. Chen, K. Wu, Nat. Chem. 8, 563–568 (2016).

3. X. Liu, Z. Zhang, L. Wang, B. I. Yakobson, and M. C. Hersam, Nat. Mater.17, 783–788 (2018).

4. X. Liu, L. Wang, S. Li, M. S. Rahn, B. I. Yakobson, and M. C. Hersam, Nat. Commun. 10, 1642 (2019).

Here we present a charge control of native defects in the bulk transition metal dichalcogenide, MoTe₂ by scanning tunneling microscopy (STM). Bulk MoTe₂ was cleaved at room temperature in ultrahigh vacuum and imaged with a cut PtIr tip at 9K. Native defects in the MoTe₂ are present throughout the sample and image with complex structure. In topographic imaging, the long-range protrusion of a bright defect indicates the species is charged and we image the defects at different depths below the surface. They present with an ionization feature in tunneling spectroscopic mapping which indicates that the charge state of this defect can be manipulated by the band bending caused by the tip. Voltage pulses from the tip migrate the subsurface defects to the surface layer. The migrated defects present with the same spectroscopic signature as native surface defects. We also present DFT results that we use to clarify the identification of these native defects and energy barriers for migration between layers of 2H-MoTe₂.

Covalent Organic Frameworks (COFs) and similar materials synthesized from bottom-up procedures grant scientists a means to customize a 2D material’s final properties from its initial precursors. The dimethylmethylene-bridged triphenylamine (DTPA) COF is an excellent example of the unique systems one can fabricate with these techniques¹. The DTPA COF is initially synthesized on Au(111) or Ag(111) which facilitates crystalline growth through Ullman-type coupling. With an even number of electrons per unit cell, theory predicts a semiconducting electronic structure for the COF. Further heating in vacuum selectively cleaves the dimethyl groups but leaves the framework intact. This final COF resembles an ultra-flat covalent network of triangulene molecules with enticing proposed electronic properties, which depend on how the demethylated sites are terminated. In the case of hydrogen termination at these sites, calculations indicate that the “radical” COF on a metallic substrate will be a half-metal (fully spin-polarized density of states at the fermi energy)². Using a low-temperature scanning tunneling microscope, the work outlined in this talk presents new information on the electronic structure of the DTPA COF in both its intact (methylated) and radical (demethylated) forms.

[1] Bieri, Marco, Stephan Blankenburg, Milan Kivala, Carlo A. Pignedoli, Pascal Ruffieux, Klaus Müllen, and Roman Fasel. “Surface-Supported 2D Heterotriangulene Polymers.” Chem. Commun. 47, no. 37 (2011): 10239–41. https://doi.org/10.1039/C1CC12490K .

[2] Kan, Erjun, Wei Hu, Chuanyun Xiao, Ruifeng Lu, Kaiming Deng, Jinlong Yang, and Haibin Su. “Half-Metallicity in Organic Single Porous Sheets.” Journal of the American Chemical Society 134, no. 13 (April 4, 2012): 5718–21. https://doi.org/10.1021/ja210822c .

Unlike graphene, semiconducting graphene nanoribbons poccess a band gap and they have the capability to electronic decouple the molecules from the supporting substrate. Here, we study the structural, electronic and vibrational properties of individual para-sexiphenyl molecules adsorbed on semiconducting graphene nanoribbons. The graphene nanoribbons here are grown on the surface of Au(111) using DBBA molecules as the initial building blocks. Para-sexiphenyl molecules adsorb on the graphene naribbons with their long molecular axis positioning parallel to the long axis of the graphene nanoribbons. As expected, the tunneling spectroscopy data reveal the HOMO-LUMO gap of the molecule on graphene nanoribbons much closer to their gas phase values. Moreover, teh vibration spectroscopy measurements of the molecules further provide a strong vibration mode associate with the C=C ring stretching of the molecules. This work provide single molecule level information on the electronic, vibronic and structural properties of the molecules adsorbed on vertically stacked heterostructures formed by graphene nanoribbons on Au(111) surface.

Acknowledgement: This work is supported by the DOE-BES, DE-FG02-02ER46012 grant.

Molecular flexure, and molecule-metal contact of para-sexiphenyl molecules on a Ag(111) surface are investigated by using low temperature scanning tunneling microscopy, and molecular manipulations. Atom trapping with sexiphenyl molecules is realized by laterally manipulating the molecules onto individual silver atoms and up to three silver atoms have been trapped. We also demonstrate breaking of a silver dimer into individual silver atoms by atom trapping. STM manipulation experiments show that the molecule-metal complexes formed by the atom trapping are mechanically stable. Moreover, lateral manipulation of a single seziphenyl across a Ag(111) atomic step highlights how the molecule moves across step-edges; the molecule can easily conform across the step and it recovers original configuration after the manipulation.

Acknowledgement: This woek is supported by the DOE-BES, DE-FG02-02ER46012 grant.

We recently demonstrated the first observation of a 2D room-temperature-ferromagnetic monolayer of MnGaN (2D-MnGaN) using spin-polarized scanning tunneling microscopy and spectroscopy. The sample is grown by molecular beam epitaxy on gallium nitride substrates. We resolved ferromagnetic domains using SP-STM, demonstrated magnetic hysteresis using small out-of-plane magnetic fields, observed magnetic rim states, and measured magnetic DOS profiles using tunneling spectroscopy which are in excellent agreement with the predicted spin-polarized & spin-split DOS peaks obtained from first-principles theory. This work was published online in December 2017 in Nano Letters.[1]

More recently, we are investigating the dependence of magnetization anisotropy on in-plane lattice strain. First of all, we have observed from the spectroscopy measurements that the position of the spin-polarized Mn DOS peak varies from spectrum to spectrum, ranging from -1.69 eV up to -1.22 eV (relative to E_Fermi). In order to investigate if these variations could be related to structural variations, we have also carried out theoretical calculations based on first principles for both isotropic and local anisotropic lattice strains. The isotropic strain case shows that the occupied-states Mn peak can indeed shift by many tenths of an eV if the 2D-MnGaN is strained in-plane; for example, E = -1.58 eV for the no-strain case, whereas E = -1.33 eV for tensile strain (+9.1%) and E = -2.22 eV for compressive strain (-6.0%). On the other hand, we find an opposite behavior in the local anisotropic calculation.

Using atomic resolution STM, we have also found that significant strain variations exist within the 2D-MnGaN. As compared to an ideally periodic hexagonal lattice, the 2D-MnGaN lattice displays local spacing variations, and the spacing distribution is highly non-Gaussian and may instead be characterized as tri-modal with the central peak matching closely the expected average for 2D-MnGaN of 5.52 Å, but with left and right peaks centered around 5.00 Å and 5.92 Å. Therefore, the Mn atoms, centered between Ga adatoms, are under highly varying strains, ranging from tensile to compressive.

By mapping the observed Mn peak energies onto theoretical energy-strain curves, we can then estimate the expected lattice parameters corresponding to particular energies and compare with the lattice spacing distribution. These results will be discussed as well as the additional discovery of a dependence of the spin anisotropy on the lattice strain.

[1] Yingqiao Ma, Abhijit V. Chinchore, Arthur R. Smith, María Andrea Barral, and Valeria Ferrari, Nano Letters 18, 158 (2018).