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Image of the Month
Posted Date: April 1, 2017
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(c) STM image of a branch of a fractal. The sample bias is 0.5 V and the tunnel current is 0.2 nA. (d) Differential conductivity recorded at the environment of the fractal, red curve (region (i) of (c)), and on the fractal, blue curve (region (i) of (c)), and on the fractal, blue curve (region (ii) of (c)). The sample bias for both curves is 0.5 V and the tunnel current is 0.2 nA.

Abstract
The basic science responsible for the fascinating  shapes of ice crystals and snowflakes is still not understood. Insufficient knowledge of the interaction potentials and the lack of relevant experimental access to the growth process are to blame for this failure. Here, we study the growth of fractal nanostructures in a two-dimensional  (2D) system, intercalated between mica and graphene. Based on our scanning tunneling spectroscopy data, we provide compelling evidence that these fractals are 2D ice. They grow while they are in material contact with the atmosphere at 20 ºC and without significant thermal contact to the ambient. The growth is studied in situ, in real time and space at the nanoscale. We find that the growing 2D ice nanocrystals assume a fractal shape, which is conventionally attributed to Diffusion Limited Aggregation (DLA). However, DLA requires a low mass density mother phase, in contrast to the actual currently present high mass density mother phase. Latent heat effects and consequent transport of heat and molecules are found to be the key ingredients for understanding the evolution of the snow (ice) flakes. We conclude that not the local availability of water molecules (DLA), but rather them having the locally required orientation is the key factor for incorporation into the 2D ice nanocrystal. In combination with the transport of latent heat, we attribute the evolution of fractal 2D ice nanocrystals to local temperature dependent rotation limited aggregation. The ice growth occurs under extreme supersaturation, i.e., the conditions closely resemble the natural ones for the growth of complex 2D snow (ice) flakes and we consider our findings crucial for solving the “perennial” snow (ice) flake enigma. © 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4926467]

Reference:
Journal of Chemical Physics 143, 034702 (2015)

Credits:
Pantelis Bampoulis,1,2,a) Martin H. Siekman,1  E. Stefan Kooij,1 Detlef Lohse,2 Harold J. W. Zandvliet,1  and 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 of Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands

Microscope:
RHK UHV3500 STM/AFM with Inverted Viewport

Control System:
RHK SPM1000

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Panscan Freedom SPM,  VT Beetle
Events
Event Date: March 19, 2017

March 19-24 Dresden, Germany
DPG – 2017 Spring Meeting of the Condensed Matter Section
The Conference will be held at the TU Dresden in Dresden, Germany
http://dresden17.dpg-tagungen.de

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Panscan Freedom SPM,  VT Beetle
Events
Event Date: March 13, 2017

March 13-16 New Orleans, LA
APS 2017 March Meeting
The Conference will be held at the Ernest N. Morial Convention Center in New Orleans, LA
http://www.aps.org/meetings/march/

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Image of the Month
Posted Date: March 7, 2017
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Figure 2 | Imaging silicon atoms with terahertz-driven scanning tunnelling microscopy (TD-STM). a. Topographic images of the Si(111)-(7 x 7) surface measured in constant-current mode with standard STM at Vd.c. = -0.5 V, ETHz,pk = 0 V cm-1, Id.c. = -20 pA b. and terahertz-driven STM (TD-STM) of the same region with Vd.c. = 0 V, ETHz,pk = -200 V cm-1ITHz,avg = -20 pA

Abstract
Ultrafast control of current on the atomic scale is essential for future innovations in nanoelectronics. Extremely localized transient electric fields on the nanoscale can be achieved by coupling picosecond duration terahertz pulses to metallic nanostructures. Here, we demonstrate terahertz scanning tunnelling microscopy (THz-STM) in ultrahigh vacuum as a new platform for exploring ultrafast non-equilibrium tunnelling dynamics with atomic precision. Extreme terahertz-pulse-driven tunnel currents up to 107 times larger than steady-state currents in conventional STM are used to image individual atoms on a silicon surface with 0.3 nm spacial resolution. At terahertz frequencies, the metallic-like Si(111)-(7 x 7) surface is unable to screen the electric field from the bulk, resulting in a terahertz tunnel conductance that is fundamentally different than that of the steady state. Ultrafast terahertz-induced band bending and non-equilibrium charging of surface states open new conduction pathways to the bulk, enabling extreme transient tunnel currents to flow between the tip and sample.

Public Reference:
NATURE PHYSICS: DOI: 10.1038/NPHYS4047

Credits:
Vedran Jelic1*, Krzysztof Iwaszczuk2, Peter H. Nguyen1, Christopher Rathje3, Graham J. Hornig1, Haille M. Sharum1, James R. Hoffman1, Mark R. Freeman1 and Frank A. Hegmann1*

1Department of Physics, University of Alberta, Edmonton, Alberta T6G 2E1, Canada. 2DTU Fotonik – Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark. 34th Physical Institute, University of Gottingen, 37077 Gottingen, Germany.
*e-mail: [email protected];, [email protected]

Microscope:
RHK UHV3500 STM/AFM with Inverted Viewport

Control System:
RHK SPM1000

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