Image of the Month

Design and implementation of a device based on an off-axis parabolic mirror to perform luminescence experiments in a scanning tunneling microscope

Rev. Sci. Instrum. 93, 043704

Ricardo Javier Peña Román, Yves Auad, Lucas Grasso, Lazaro A Padilha, Fernando Alvarez,
Ingrid David Barcelos, Mathieu Kociak, and Luiz Fernando Zagonel

Fig. 1 | Experimental set-up and strategy for quantifying heat transport in single-molecule junctions.

Abstract
Single-molecule junctions have been extensively used to probe properties as diverse as electrical conduction^1–3, light emission⁴, thermoelectric energy conversion^5,6, quantum interference^7,8, heat dissipation^9,10 and electronic noise¹¹ at atomic and molecular scales. However, a key quantity of current interest—the thermal conductance of single-molecule junctions—has not yet been directly experimentally determined, owing to the challenge of detecting minute heat currents at the picowatt level. Here we show that picowatt-resolution scanning probes previously developed to study the thermal conductance of single-metal-atom junctions¹², when used in conjunction with a time-averaging measurement scheme to increase the signal-to-noise ratio, also allow quantification of the much lower thermal conductance of single-molecule junctions. Our experiments on prototypical Au–alkanedithiol–Au junctions containing two to ten carbon atoms confirm that thermal conductance is to a first approximation independent of molecular length, consistent with detailed ab initio simulations. We anticipate that our approach will enable systematic exploration of thermal transport in many other one-dimensional systems, such as short molecules and polymer chains, for which computational predictions of thermal conductance^13–16 have remained experimentally inaccessible.

Reference:
Nature 572, 628 – 633 (2019) doi.org/10.1038/s41586-019-1420-z

Credits:
Longji Cui^1,6, Sunghoon Hur¹, Zico Alaia Akbar², Jan C. Klöckner^3,4, Wonho Jeong¹, Fabian Pauly^3,4*, Sung-Yeon Jang^2,7*, Pramod Reddy^1,5* & Edgar Meyhofer^1*

¹ Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, USA.
² Department of Chemistry, Kookmin University, Seoul, South Korea.
³ Okinawa Institute of Science and Technology Graduate University, Onna-son, Okinawa, Japan.
⁴ Department of Physics, University of Konstanz, Konstanz, Germany.
⁵ Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, USA.
⁶ Present address: Smalley-Curl Institute and Department of Physics and Astronomy, Rice University, Houston, TX, USA.
⁷ Present address: Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, South Korea.

*e-mail: [email protected]; [email protected]; [email protected]; [email protected]

Microscope:
UHV Beetle 750 AFM-STM

FIG. 1. STM images of (a) pure Au (1.0 MLe), (b) Au (1.0 MLe) and subsequently Rh (1.0 MLe), (c) pure Rh (1.0 MLe), (d) Rh (1.0 MLe) and subsequently Au (0.2 MLe) deposited on thin-film Al2O3/NiAl(100) at 300 K. The insets of each figure show histograms of diameters and heights of the clusters. The grey bars in the histograms of (b) and (d) correspond to the STM images and the red bars are from those of (a) and (c), respectively, shown side by side to demonstrate the variation of the size distribution.

Abstract
The surface structures and compositions of Au–Rh bimetallic nanoclusters on an ordered thin film of Al2O3/NiAl(100) were investigated, primarily with infrared reflection absorption spectra and temperature-programmed desorption of CO as a probe molecule under ultrahigh-vacuum conditions and calculations based on density-functional theory. The bimetallic clusters were formed by sequen- tial deposition of vapors of Au and Rh onto Al2O3/NiAl(100) at 300 K. Alloying in the clusters was active and proceeded toward a specific structure—a fcc phase, (100) orientation, and Rh core-Au shell structure, regardless of the order of metal deposition. For Au clusters incorporating deposited Rh, the Au atoms remained at the cluster surface through position exchange and became less coordinated; for deposition in reverse order, deposited Au simply decorated the surfaces of Rh clusters. Both adsorption energy and infrared absorption intensity were enhanced for CO on Au sites of the bimetallic clusters; both of them are associated with the bonding to Rh and also a decreased coordination number of CO- binding Au. These enhancements can thus serve as a fingerprint for alloying and atomic inter-diffusion in similar bimetallic systems.

Reference:
The Journal of Chemical Physics 147, 044704 (2017); doi: 10.1063/1.4995598

Credits:
Hsuan Lee,1 Zhen-He Liao,1 Po-Wei Hsu,1 Ting-Chieh Hung,1 Yu-Cheng Wu,1 Yuwei Lin,2 Jeng-Han Wang,2,a) and Meng-Fan Luo1,a)

1 Department of Physics, National Central University, 300 Jhongda Road, Taoyuan 32001, Taiwan

2 Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan 

a) Authors to whom correspondence should be addressed: [email protected]. edu.tw and [email protected]

Microscope:
Beetle UHV 300

FIG. 2. Model of 8T and 7T molecules from Fig. 1 matched to the Au(111) surface lattice. ((a)-(d)) STM images [set point 100 mV, 5 pA] of sub-areas from Figs. 1(a)-1(c). ((e)-(h)) Models of oligothiophene molecules from (a)-(d) overlaid on the Au(111) lattice. Crystallographic directions are indi- cated. Black dashed circles indicate the van der Waals radii of the hydrogen atoms. Red dashed circles for the highlighted molecules indicate S atoms on or near Au top-sites (as opposed to bridging or hollow sites).

Abstract
We present scanning tunneling microscopy and spectroscopy (STM/STS) investigations of the electronic structures of di erent alkyl-substituted oligothiophenes on the Au(111) surface. STM imaging showed that on Au(111), oligothiophenes adopted distinct straight and bent conformations. By combining STS maps with STM images, we visualize, in real space, particle-in-a-box-like oligothiophene molecular orbitals. We demonstrate that di erent planar conformers with signi cant geometrical distortions of oligothiophene backbones surprisingly exhibit very similar electronic structures, indicating a low degree of conformation-induced electronic disorder. The agreement of these results with gas-phase density functional theory calculations implies that the oligothiophene interaction with the Au(111) surface is generally insensitive to molecular conformation.

Reference:
The Journal of Chemical Physics 144, 194703 (2016); doi: 10.1063/1.4949765

Credits:
Benjamen N. Taber,1 Dmitry A. Kislitsyn,1 Christian F. Gervasi,1 Jon M. Mills,1 Ariel E. Rosenfield,1 Lei Zhang,2 Stefan C. B. Mannsfeld,3 James S. Prell,1 Alejandro L. Briseno,2 and George V. Nazin1

1. Department of Chemistry and Biochemistry, Materials Science Institute, Oregon Center for Optical, Molecular and Quantum Science, University of Oregon, 1253 University of Oregon, Eugene, Oregon 97403, USA

2. Department of Polymer Science and Engineering, Silvio O. Conte National Center for Polymer Research, University of Massachusetts-Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, USA


3. Center for Advancing Electronics Dresden, Dresden University of Technology, 01062 Dresden, Germany

Microscope:
Customized RHK PanScan Freedom Kit with RHK R9 Controller

Fig. 4 Overview of the supramolecular phases and the derived structural models for 2HTPTBP on Cu(111) at different coverages (a–c: relatively low coverage; d–f: medium coverage; g–i: monolayer coverage; all the STM images were measured at RT). The red and yellow ovals indicate the individual molecules with different orientations. The intermolecular T-type-like interactions and p–p stacking interactions in the structural models are highlighted by green lines and black parallel lines, respectively. The van der Waals interactions between the phenyl groups of two neighbouring molecules are marked by green dashed lines. Tunnelling parameters: (a) U = 1.0 V, I = 30 pA; (b) U = 1.6 V, I = 30 pA; (d, e, g and h) U = 1.1 V, I = 30 pA.

Abstract
The adsorption behaviour of 2H-5,10,15,20-tetraphenyltetrabenzoporphyrin (2HTPTBP) on different metal surfaces, i.e., Ag(111), Cu(111), Cu(110), and Cu(110)–(2 ? 1)O was investigated by scanning tunnelling microscopy at room temperature. The adsorption of 2HTPTBP on Ag(111) leads to the formation of a well- ordered two-dimensional (2D) island structure due to the mutual stabilization through the intermolecular p–p stacking and T-type-like interactions of phenyl and benzene substituents of neighboring molecules. For 2HTPTBP on Cu(111), the formed 2D supramolecular structures exhibit a coverage-dependent behaviour, which can be understood from the interplay of molecule–substrate and molecule–molecule interactions. In contrast, on Cu(110) the 2HTPTBP molecules form dispersed one-dimensional (1D) molecular chains along the [11%0] direction of the substrate due to relatively strong attractive molecule– substrate interactions. Furthermore, we demonstrate that the reconstruction of the Cu(110) surface by oxygen atoms yields a change in dimensionality of the resulting nanostructures from 1D on Cu(110) to 2D on (2 ? 1) oxygen-reconstructed Cu(110), induced by a decreased molecule–substrate interaction combined with attractive molecule–molecule interactions. This comprehensive study on these prototypical systems enables us to deepen the understanding of the particular role of the substrate concerning the adsorption behavior of organic molecules on metal surfaces and thus to tweak the ordering in functional molecular architectures.

Reference:
Physical Chemistry Chemical Physics 19.30 (2017): 20281-20289

Credits:
Liang Zhang, ab Michael Lepper, ab Michael Stark,ab Teresa Menzel,ab
Dominik Lungerich,bc Norbert Jux,bc Wolfgang Hieringer,bd Hans-Peter Steinru ̈ckab and Hubertus Marbach*ab

a Lehrstuhl fu ̈r Physikalische Chemie II, Universita ̈t Erlangen-Nu ̈rnberg, Egerlandstr. 3, 91058 Erlangen, Germany. E-mail: [email protected]

b Interdisciplinary Center for Molecular Materials (ICMM), Universita ̈t Erlangen-Nu ̈rnberg, Germany

c Lehrstuhl fu ̈r Organische Chemie II, Universita ̈t Erlangen-Nu ̈rnberg, Henkestr. 42, 91054 Erlangen, Germany

d Lehrstuhl fu ̈r Theoretische Chemie, Universita ̈t Erlangen-Nu ̈rnberg, Egerlandstr. 3, 91058 Erlangen, Germany

† Electronic supplementary information (ESI) available: The structure models of 2HTPTBP on Cu(111), the STM images of 2HTPTBP on Cu(110) after annealing and the atomic Cartesian coordinates for the calculated gas phase model. See DOI: 10.1039/c7cp03731g

Microscope:
UHV VT Beetle 300

Figure 3. STM images after exposing Rh(111) to AO: (A) θO,total = 6.4 ML at Tdep = 700 K; (B) θO,total = 2.9 ML at Tdep = 350 K followed by 600 s anneal at 700 K; (C) θO,total = 0.9 ML at Tdep = 350 K followed by 600 s anneal at 700 K. Insets are LEED patterns (62 eV) taken after deposition. STM images were obtained at 30 K and conditions were (L to R) (A) 100 mV, 137 pA; 181 mV, 170 pA; 140 mV, 153 pA; (B) 50 mV, 400 pA; 70 mV, 410 pA; 50 mV, 400 pA; (C) 20 mV, 200 pA; 20 mV, 150 pA; 20 mV, 150 pA.

Abstract
Recent studies have shown the importance of oxide surfaces in heterogeneously catalyzed reactions. Because of the difficulties in reproducibly preparing oxidized metal surfaces, it is often unclear what species are thermodynamically stable and what factors effect the oxide formation process. In this work, we show that the thermodynamically stable phases on Rh(111) after exposure to atomic oxygen are the (2×1)- O adlayer and the trilayer surface oxide, RhO2. Formation of RhO2 was facilitated by surface defects and elevated concentrations of dissolved O atoms in the subsurface region. As the concentration of subsurface O atoms decreased, the coverage of RhO2 decreased so that only the (2×1)- O adlayer was present on the surface. The importance of subsurface oxygen species in RhO2 formation and stability indicates a complex relationship between surface structure and subsurface oxygen concentration.

Reference:
The Journal of Physical Chemistry C 121.19 (2017): 10470-10475

Credits:
Rachael G. Farber,† Marie E. Turano,† Eleanor C. N. Oskorep,† Noelle T. Wands,† Erin V. Iski,‡ and Daniel R. Killelea*,†

†Department of Chemistry & Biochemistry, Loyola University Chicago, 1068 West Sheridan Road, Chicago, Illinois 60660, United States

‡Department of Chemistry and Biochemistry, The University of Tulsa, 800 South Tucker Drive, Tulsa, Oklahoma 74104, United States

Microscope:
Closed Cycle Pan Freedom System

Figure 3. STM images of quasi-freestanding WSe2 islands. (a) STM image of 90 × 75 Å2 area of elevated
1 ML island obtained at 2 V sample bias. (b) Image of the same area at 3 V bias. (c) Cross-section of type-B (multi-ring) pattern from (a). (d) Cross-section of type-A (single-ring) pattern from (a). The STM cross- sections are oriented perpendicular to atomic rows, and the horizontal axes are normalized to a0. The central minima in (c,d) have slightly different shapes due to different contributions of cosine modes (see Discussion part and Supplementary Note 2). (e) The larger scale, 260 × 260 Å2, STM image of phonon interference patterns on elevated 1 ML island. The image uses gradient contrast. One of type-B and one of type-A patterns are schematically surrounded by dotted lines. For gradient contrast, the missing half-rings are less visible. Bright- contrast features originate from residual contaminating particles. (Left inset) The left inset shows different absorption sites for defects, H-site vs. TM-site, that may also cause type-A vs. type-B standing wave patterns. (Right inset) STM image in the right inset clarifies the horizontal axis units in (c,d) and the orientation of crystal axes in (a,b,e). The pattern on this STM image (surrounded by dotted line type-C pattern) only contains a broad central minimum.

Abstract
Using quantum tunneling of electrons into vibrating surface atoms, phonon oscillations can be observed on the atomic scale. Phonon interference patterns with unusually large signal amplitudes have been revealed by scanning tunneling microscopy in intercalated van der Waals heterostructures. Our results show that the effective radius of these phonon quasi-bound states, the real-space distribution of phonon standing wave amplitudes, the scattering phase shifts, and the nonlinear intermode coupling strongly depend on the presence of defect-induced scattering resonance. The observed coherence of these quasi-bound states most likely arises from phase- and frequency-synchronized dynamics of
all phonon modes, and indicates the formation of many-body condensate of optical phonons around resonant defects. We found that increasing the strength of the scattering resonance causes the increase of the condensate droplet radius without affecting the condensate fraction inside it. The condensate can be observed at room temperature.

Reference:
Scientific Reports 7 (2017): 43214

Credits:
Igor Altfeder¹, Andrey A. Voevodin^1,2, Michael H. Check¹, Sarah M. Eichfeld³, Joshua A. Robinson³ & Alexander V. Balatsky^4,5

¹Nanoelectronic Materials Branch, Air Force Research Laboratory, Wright Patterson AFB, OH 45433, USA.
²Department of Materials Science and Engineering, University of North Texas, Denton, Texas 76203, USA.
³Department of Materials Science and Engineering and The Center for Two-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.
⁴Institute for Materials Science, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
⁵Nordita, Center for Quantum Materials, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 10691 Stockholm, Sweden. Correspondence and requests for materials should be addressed to I.A. (email: [email protected])

Microscope:
UHV Beetle 300 STM

Figure 2. Scanning tunneling microscopy of the Graphene-Ice-Mica interface. (a) UHV STM topography (190 × 190 nm2) of a few layers graphene deposited on mica recorded at 0.2 V and 100 pA. Ice crystals (darker regions) are observed intercalated between graphene and mica surrounded by two water layers (brighter region). (b) A high resolution image (17 × 17 nm2) at the edges between an ice crystal and two water layers. A ripple-like structure is observed. (c) An atomic resolution image (6 × 6 nm2) clearly showing the ripple-like structure of graphene. (d) FFT of the hills (red circle in panel (c)) and valleys (green circle in panel (c)) of the rippled graphene surface and the higher edges of the fractal (blue circle in panel (b)).

Abstract
The distribution of potassium (K⁺) ions on air-cleaved mica is important in many interfacial phenomena such as crystal growth, self-assembly and charge transfer on mica. However, due to experimental limitations to nondestructively probe single ions and ionic domains, their exact lateral organization
is yet unknown. We show, by the use of graphene as an ultra-thin protective coating and scanning probe microscopies, that single potassium ions form ordered structures that are covered by an ice layer. The K⁺ ions prefer to minimize the number of nearest neighbour K⁺ ions by forming row-like structures as well as small domains. This trend is a result of repulsive ionic forces between adjacent ions, weakened due to screening by the surrounding water molecules. Using high resolution conductive atomic force microscopy maps, the local conductance of the graphene is measured, revealing a direct correlation between the K⁺ distribution and the structure of the ice layer. Our results shed light on the local distribution of ions on the air-cleaved mica, solving a long-standing enigma. They also provide a detailed understanding of charge transfer from the ionic domains towards graphene.

Reference:
Scientific reports 7 (2017): 43451.

Credits:
Pantelis Bampoulis^1,2,*, Kai Sotthewes^1,*, Martin H. Siekman¹, Harold J. W. Zandvliet¹ & Bene Poelsema¹

¹Physics of Interfaces and Nanomaterials, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands.
²Physics of Fluids and J.M. Burgers Centre for Fluid Mechanics, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands.
^*These authors contributed equally to this work. Correspondence and requests for materials should be addressed to P.B. (email: [email protected])

Microscope:
UHV Beetle 3000 STM