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1.
Nano Lett ; 19(7): 4594-4600, 2019 Jul 10.
Artigo em Inglês | MEDLINE | ID: mdl-31251073

RESUMO

When graphene is placed on a crystalline surface, the periodic structures within the layers superimpose and moiré superlattices form. Small lattice rotations between the two materials in contact strongly modify the moiré lattice parameter, upon which many electronic, vibrational, and chemical properties depend. While precise adjustment of the relative orientation in the degree- and sub-degree-range can be achieved via careful deterministic transfer of graphene, we report on the spontaneous reorientation of graphene on a metallic substrate, Ir(111). We find that selecting a substrate temperature between 1530 and 1000 K during the growth of graphene leads to distinct relative rotational angles of 0°, ± 0.6°, ±1.1°, and ±1.7°. When modeling the moiré superlattices as two-dimensional coincidence networks, we can ascribe the observed rotations to favorable low-strain graphene structures. The dissimilar thermal expansion of the substrate and graphene is regarded as an effective compressive biaxial pressure that is more easily accommodated in graphene by small rotations rather than by compression.

2.
Ultramicroscopy ; 162: 17-24, 2016 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-26720438

RESUMO

Helium Ion Microscopy is known for its surface sensitivity and high lateral resolution. Here, we present results of a Helium Ion Microscopy based investigation of a surface confined alloy of Ag on Pt(111). Based on a change of the work function of 25meV across the atomically flat terraces we can distinguish Pt rich from Pt poor areas and visualize the single atomic layer high steps between the terraces. Furthermore, dechanneling contrast has been utilized to measure the periodicity of the hcp/fcc pattern formed in the 2-3 layers thick Ag/Pt alloy film. A periodicity of 6.65nm along the ⟨112⟩ surface direction has been measured. In terms of crystallography a hcp domain is obtained through a lateral displacement of a part of the outermost layer by 1/√3 of a nearest neighbor spacing along ⟨112⟩. This periodicity is measured with atomic precision: coincidence between the Ag and the Pt lattices is observed for 23 Ag atoms on 24 Pt atoms. The findings are perfectly in line with results obtained with Low Energy Electron Microscopy and Phase Contrast Atomic Force Microscopy.

3.
Beilstein J Nanotechnol ; 4: 453-60, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-23946914

RESUMO

Helium ion microscopy (HIM) was used to investigate the interaction of a focused He(+) ion beam with energies of several tens of kiloelectronvolts with metals. HIM is usually applied for the visualization of materials with extreme surface sensitivity and resolution. However, the use of high ion fluences can lead to significant sample modifications. We have characterized the changes caused by a focused He(+) ion beam at normal incidence to the Au{111} surface as a function of ion fluence and energy. Under the influence of the beam a periodic surface nanopattern develops. The periodicity of the pattern shows a power-law dependence on the ion fluence. Simultaneously, helium implantation occurs. Depending on the fluence and primary energy, porous nanostructures or large blisters form on the sample surface. The growth of the helium bubbles responsible for this effect is discussed.

4.
Phys Chem Chem Phys ; 15(14): 5007-16, 2013 Apr 14.
Artigo em Inglês | MEDLINE | ID: mdl-23443309

RESUMO

The molecular arrangement of 4,4'-biphenyldicarboxylic acid (BDA) on Cu(001) has been studied at high coverage and relatively high temperature (~400 K) using Low Energy Electron Microscopy, LEEM, and selected area diffraction, µLEED. Next to the previously reported c(8 × 8) structure, we also observe a compressed phase with a [structure: see text] superstructure in matrix notation. All four equivalent (rotational and mirror) domains are equally populated. Both the c(8 × 8) and the compressed phase are confined to the first layer and the latter has a 14% higher density compared to the c(8 × 8) phase. Remarkably, this compressed phase is stable only during deposition and decays after interruption of the deposition. Apparently, the density of physisorbed admolecules on top of the c(8 × 8) layer has to be above a relevant threshold to allow the formation of the compressed phase.


Assuntos
Compostos de Bifenilo/química , Compostos de Bifenilo/síntese química , Cobre/química , Ácidos Dicarboxílicos/química , Ácidos Dicarboxílicos/síntese química , Modelos Moleculares , Estrutura Molecular , Propriedades de Superfície , Temperatura
5.
Small ; 9(5): 711-5, 2013 Mar 11.
Artigo em Inglês | MEDLINE | ID: mdl-23161747

RESUMO

Large area low-cost patterning is a challenging problem in graphene research. A resist-free, single-step, large area and cost effective soft lithographic patterning strategy is presented for graphene. The technique is applicable on any arbitrary substrate that needs to be covered with a graphene film and provides a viable route to large-area patterning of graphene for device applications.

6.
Phys Rev Lett ; 110(7): 076101, 2013 Feb 15.
Artigo em Inglês | MEDLINE | ID: mdl-25166384

RESUMO

We have used low energy electron microscopy to demonstrate how the interaction of 4,4'-biphenyldicarboxylic acid (BDA) molecules with (steps on) the Cu(001) surface determines the structure of supramolecular BDA networks on a mesoscopic length scale. Our in situ real time observations reveal that steps are permeable to individual molecules but that the change in crystal registry between different layers of the Cu substrate causes them to be completely impermeable to condensed BDA domains. The resulting growth instabilities determine the evolution of the domain shape and include a novel Mullins-Sekerka-type growth instability that is characterized by high growth rates along, instead of perpendicular to, the Cu steps. This growth instability is responsible for the majority of residual defects in the BDA networks.

7.
Phys Rev Lett ; 109(1): 016101, 2012 Jul 06.
Artigo em Inglês | MEDLINE | ID: mdl-23031116

RESUMO

We present a low-energy electron microscopy study of nucleation and growth of BDA on Cu(001) at low supersaturation. At sufficiently high coverage, a dilute BDA phase coexists with c(8×8) crystallites. The real-time microscopic information allows a direct visualization of near-critical nuclei, determination of the supersaturation and the line tension of the crystallites, and, thus, derivation of the Gibbs free energy for nucleation. The resulting critical nucleus size nicely agrees with the measured value. Nuclei up to 4-6 times larger still decay with finite probability, urging reconsideration of the classic perception of a critical nucleus.

8.
Beilstein J Nanotechnol ; 3: 501-6, 2012.
Artigo em Inglês | MEDLINE | ID: mdl-23019544

RESUMO

BACKGROUND: The unique surface sensitivity and the high resolution that can be achieved with helium ion microscopy make it a competitive technique for modern materials characterization. As in other techniques that make use of a charged particle beam, channeling through the crystal structure of the bulk of the material can occur. RESULTS: Here, we demonstrate how this bulk phenomenon affects secondary electron images that predominantly contain surface information. In addition, we will show how it can be used to obtain crystallographic information. We will discuss the origin of channeling contrast in secondary electron images, illustrate this with experiments, and develop a simple geometric model to predict channeling maxima. CONCLUSION: Channeling plays an important role in helium ion microscopy and has to be taken into account when trying to achieve maximum image quality in backscattered helium images as well as secondary electron images. Secondary electron images can be used to extract crystallographic information from bulk samples as well as from thin surface layers, in a straightforward manner.

9.
Beilstein J Nanotechnol ; 3: 507-12, 2012.
Artigo em Inglês | MEDLINE | ID: mdl-23019545

RESUMO

BACKGROUND: Helium ion microscopy is a new high-performance alternative to classical scanning electron microscopy. It provides superior resolution and high surface sensitivity by using secondary electrons. RESULTS: We report on a new contrast mechanism that extends the high surface sensitivity that is usually achieved in secondary electron images, to backscattered helium images. We demonstrate how thin organic and inorganic layers as well as self-assembled monolayers can be visualized on heavier element substrates by changes in the backscatter yield. Thin layers of light elements on heavy substrates should have a negligible direct influence on backscatter yields. However, using simple geometric calculations of the opaque crystal fraction, the contrast that is observed in the images can be interpreted in terms of changes in the channeling probability. CONCLUSION: The suppression of ion channeling into crystalline matter by adsorbed thin films provides a new contrast mechanism for HIM. This dechanneling contrast is particularly well suited for the visualization of ultrathin layers of light elements on heavier substrates. Our results also highlight the importance of proper vacuum conditions for channeling-based experimental methods.

10.
Acta Biomater ; 8(8): 2969-77, 2012 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-22522130

RESUMO

Numerous studies have shown that the physicochemical properties of biomaterials can control cell activity. Cell adhesion, proliferation, differentiation as well as tissue formation in vivo can be tuned by properties such as the porosity, surface micro- and nanoscale topography and chemical composition of biomaterials. This concept is very appealing for tissue engineering since instructive properties in bioactive materials can be more economical and time efficient than traditional strategies of cell pre-differentiation in vitro prior to implantation. The biomaterial surface, which is easy to modify due to its accessibility, may provide the necessary signals to elicit a certain cellular behavior. Here, we used gas plasma technology at atmospheric pressure to modify the physicochemical properties of polylactic acid and analyzed how this influenced pre-osteoblast proliferation and differentiation. Tetramethylsilane and 3-aminopropyl-trimethoxysilane with helium as a carrier gas or a mixture of nitrogen and hydrogen were discharged to polylactic acid discs to create different surface chemical compositions, hydrophobicity and microscale topographies. Such modifications influenced protein adsorption and pre-osteoblast cell adhesion, proliferation and osteogenic differentiation. Furthermore polylactic acid treated with tetramethylsilane enhanced osteogenic differentiation compared to the other surfaces. This promising surface modification could be further explored for potential development of bone graft substitutes.


Assuntos
Diferenciação Celular/efeitos dos fármacos , Osteoblastos/citologia , Osteoblastos/efeitos dos fármacos , Osteogênese/efeitos dos fármacos , Gases em Plasma/farmacologia , Adsorção/efeitos dos fármacos , Animais , Pressão Atmosférica , Materiais Biocompatíveis/farmacologia , Adesão Celular/efeitos dos fármacos , Linhagem Celular , Proliferação de Células/efeitos dos fármacos , Regulação da Expressão Gênica/efeitos dos fármacos , Interações Hidrofóbicas e Hidrofílicas/efeitos dos fármacos , Ácido Láctico/farmacologia , Camundongos , Microscopia de Força Atômica , Osteoblastos/metabolismo , Poliésteres , Polímeros/farmacologia , Soroalbumina Bovina/metabolismo , Propriedades de Superfície/efeitos dos fármacos
11.
Surf Sci ; 606(3-4): 475-480, 2012 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-22308005

RESUMO

The growth of para-sexiphenyl (6P) thin films as a function of substrate temperature on Ir{111} supported graphene flakes has been studied in real-time with Low Energy Electron Microscopy (LEEM). Micro Low Energy Electron Diffraction (µLEED) has been used to determine the structure of the different 6P features formed on the surface. We observe the nucleation and growth of a wetting layer consisting of lying molecules in the initial stages of growth. Graphene defects - wrinkles - are found to be preferential sites for the nucleation of the wetting layer and of the 6P needles that grow on top of the wetting layer in the later stages of deposition. The molecular structure of the wetting layer and needles is found to be similar. As a result, only a limited number of growth directions are observed for the needles. In contrast, on the bare Ir{111} surface 6P molecules assume an upright orientation. The formation of ramified islands is observed on the bare Ir{111} surface at 320 K and 352 K, whereas at 405 K the formation of a continuous layer of upright standing molecules growing in a step flow like manner is observed.

12.
Langmuir ; 28(5): 3045-52, 2012 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-22229783

RESUMO

A simple methodology to fabricate micrometer- and nanometer-scale patterned surfaces with multiple chemical functionalities is presented. Patterns with lateral dimensions down to 110 nm were made. The fabrication process involves multistep gas-phase patterning of amine, thiol, alkyl, and fluorinated alkyl-functional organosilane molecules using PDMS molds as shadow masks. Also, a combination process of channel diffused plasma etching of organosilane molecular thin films in combination with masked gas-phase deposition to fabricate multilength scale, multifunctional surfaces is demonstrated.

13.
Nano Lett ; 12(2): 678-82, 2012 Feb 08.
Artigo em Inglês | MEDLINE | ID: mdl-22175792

RESUMO

Following graphene growth by thermal decomposition of ethylene on Ir(111) at high temperatures we analyzed the strain state and the wrinkle formation kinetics as function of temperature. Using the moiré spot separation in a low energy electron diffraction pattern as a magnifying mechanism for the difference in the lattice parameters between Ir and graphene, we achieved an unrivaled relative precision of ±0.1 pm for the graphene lattice parameter. Our data reveals a characteristic hysteresis of the graphene lattice parameter that is explained by the interplay of reversible wrinkle formation and film strain. We show that graphene on Ir(111) always exhibits residual compressive strain at room temperature. Our results provide important guidelines for strategies to avoid wrinkling.


Assuntos
Grafite/química , Irídio/química , Cinética , Tamanho da Partícula , Propriedades de Superfície , Temperatura
14.
Phys Rev Lett ; 107(17): 176102, 2011 Oct 21.
Artigo em Inglês | MEDLINE | ID: mdl-22107543

RESUMO

The quantum-size effect (QSE) driven growth of Bi film structures on Ni(111) was studied in situ using low energy electron microscopy and selective area low energy electron diffraction (µLEED). Domains with a (3×3), [(3)(1)(-1)(2)], and (7×7) film structure are found with a height of 3, 5, and 7 atomic layers, respectively. A comparison of I/V-µLEED curves with tensor LEED calculations shows perfectly accommodated Fermi wavelengths, indicative that not only the quantized height, but also the film structure is driven by QSE.

15.
Phys Rev Lett ; 107(13): 136103, 2011 Sep 23.
Artigo em Inglês | MEDLINE | ID: mdl-22026876

RESUMO

With their low surface free energy, lead films tend to wet surfaces. However, quantum size effects (QSE) often lead to islands with distinct preferred heights. We study thin lead films on Ni(111) using low energy electron microscopy and selected area low energy electron diffraction. Indeed, the grown lead mesas show distinct evidence for QSE's. At about 526 K metastable mesas reshape into hemispheres within milliseconds, driven by a huge reduction in interfacial free energy. The underlying diffusion rate is many orders of magnitude faster than expected for lead on bulk lead.

16.
J Chem Phys ; 135(12): 124706, 2011 Sep 28.
Artigo em Inglês | MEDLINE | ID: mdl-21974551

RESUMO

The growth of 4,4(')-biphenyldicarboxylic acid (BDA) on Cu(001) has been studied using low energy electron microscopy and selective area low energy electron diffraction. The emergence of large islands and hydrogen bonding to perpendicularly oriented, adjacent molecules is confirmed. The two benzene rings of adsorbed BDA are twisted along the molecular axis. Unconventional growth of the domains, followed by a second nucleation stage, is observed at room temperature. This unanticipated feature is attributed to the accumulation of stress in the islands. Ostwald ripening in the films and the decay of BDA domains at 448 K exhibits features that are consistent with diffusion limited behavior.


Assuntos
Compostos de Bifenilo/química , Cobre/química , Ácidos Dicarboxílicos/química , Temperatura , Microscopia Eletrônica , Estrutura Molecular , Tamanho da Partícula , Propriedades de Superfície
17.
Nano Lett ; 11(2): 333-7, 2011 Feb 09.
Artigo em Inglês | MEDLINE | ID: mdl-21207968

RESUMO

High-quality thin films of conjugated molecules with smooth interfaces are important to assist the advent of organic electronics. Here, we report on the layer-by-layer growth of the organic semiconductor molecule p-sexiphenyl (6P) on the transparent electrode material graphene. Low energy electron microscopy and micro low energy electron diffraction reveal the morphological and structural evolution of the thin film. The layer-by-layer growth of 6P on graphene proceeds by subsequent adding of {111} layers.


Assuntos
Cristalização/métodos , Grafite/química , Membranas Artificiais , Nanoestruturas/química , Nanotecnologia/instrumentação , Compostos Orgânicos/química , Semicondutores , Desenho de Equipamento , Análise de Falha de Equipamento , Nanoestruturas/ultraestrutura , Tamanho da Partícula
18.
Nano Lett ; 9(5): 1733-6, 2009 May.
Artigo em Inglês | MEDLINE | ID: mdl-18781806

RESUMO

We demonstrate the feasibility of controlling an atomic scale mechanical device by an external electrical signal. On a germanium substrate, a switching motion of pairs of atoms is induced by electrons that are directly injected into the atoms with a scanning tunneling microscope tip. By precisely controlling the tip current and distance we make two atom pairs behave like the flippers of an atomic-sized pinball machine. This atomic scale mechanical device exhibits six different configurations.

19.
Phys Rev Lett ; 97(26): 266104, 2006 Dec 31.
Artigo em Inglês | MEDLINE | ID: mdl-17280433

RESUMO

The dynamic behavior of surface dimers on Ge(001) has been studied by positioning the tip of a scanning tunneling microscope over single flip-flopping dimers and measuring the tunneling current as a function of time. We observe that not just symmetric, but also asymmetric appearing dimers exhibit flip-flop motion. The dynamics of flip-flopping dimers can be used to sensitively gauge the local potential landscape of the surface. Through a spatial and time-resolved measurement of the flip-flop frequency of the dimers, local strain fields near surface defects can be accurately probed.

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