Giant Surface Stress in Heteroepitaxial Films: Invalidation of a Classical Rule in Epitaxy

Grossmann, Alexander; Erley, W.; Hannon, JB; Ibach, H.

doi:10.1103/physrevlett.77.127

Cited by 95 publications

(49 citation statements)

References 22 publications

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“…Nor can the gravimetric deflection due to the molecular loading, calculated to be ≈ 1 mV, account for the observed bending. Based on both the permanent nature of the deflection and its magnitude, we attribute the sensor response to changes in surface stress ∆σ [21][22][23].…”

Section: Resultsmentioning

confidence: 99%

Surface stress in the self-assembly of alkanethiols on gold probed .by a force microscopy technique

Berger

Delamarche

Lang

et al. 1998

Applied Physics A: Materials Science & Processing

131

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The extreme sensitivity of scanning force microscopy allows its application to be extended well beyond the probing of surfaces. We discuss micromechanical force microscope cantilevers, with a thin Au receptor layer on one side, as sensors for gas-phase adsorption of alkanethiols, which self-organize into well-ordered, densely packed films. Quantitative data show that surface stress develops during the self-assembly process for alkanethiols of different chain lengths. In particular, we present an analysis of the kinetics of the replacement process of adsorbates by butanethiol molecules, details of the acquisition analysis, and a quantitative calibration technique.Numerous local tip-sample interactions are used to obtain information on sample surface properties such as topography, conductivity, elasticity, friction, and magnetic responses. Recently, the scanning force microscope (SFM) tip has been used to probe the stretching and unfolding of individual molecules [1]. The alternative concept to using the tip of the cantilever as a local sensing element that is scanned over the surface is to use the entire cantilever surface area as a receptor. This technique constitutes a new avenue for further experiments to gather information on specific and nonspecific interactions and reactions on the nanoscale.One concept is the use of "bimetallic" cantilevers to probe temperature changes. This concept was pioneered to follow heat evolution during the catalytic reaction of O 2 and H 2 to form water on a thin Pt layer evaporated on an SFM cantilever [2]. The bimetallic technique was also applied to probe local temperature differences along a biased resistor [3]. It also enabled photothermal spectroscopy to be conducted on a picogram quantity of material [4]. Signals from the cantilever response can be detected with the high sensitivity, speed, and bandwidth of conventional SFM technique. In addition to probing subtle temperature differences, a cantilever-type sensor can additionally be used as a transducer for femtoscale effects in science [5][6][7][8][9][10][11][12][13]. We applied the micromechanical sensor technique to probe the self-organization of alkanethiols -HS−(CH 2 ) n−1 −CH 3 for n = 4, 6, 8, 12, and 14, where n is the number of carbon atoms in the alkyl chain -on Au [14,15]. Self-assembled monolayers (SAMs) are used in applications such as microcontact printing [16] and voltametric microsensors [17], and have recently been applied to molecular host-guest recognition [18]. ExperimentalOur measurement setup, outlined schematically in Fig. 1, is based on a NanoScope II SFM head (Digital Instruments, Santa Barbara CA, USA) which uses the laser beam deflection technique to detect sensor deflection ∆z down to sub-Å levels. The normalized voltage difference ∆V = (V 1 − V 2 )(V 1 + V 2 ) −1 of two segments of a positionsensitive detector (PSD) is converted into a 12-bit value in an analog-to-digital converter and is displayed continuously on a monitor as well as being stored on a computer hard disk. All V-shaped SiN x senso...

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Section: Resultsmentioning

confidence: 99%

Surface stress in the self-assembly of alkanethiols on gold probed .by a force microscopy technique

Berger

Delamarche

Lang

et al. 1998

Applied Physics A: Materials Science & Processing

131

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“…We note that interface mixing has been suggested for the relaxation of the large surface stress in the present system. 29 The quantitative details of Ref. 29 had to be corrected, [30][31][32] however, the experimental findings presented here put these results in a new light.…”

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confidence: 86%

Interface-confined mixing and buried partial dislocations for Ag bilayer on Pt(111)

et al. 2012

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The trigonal strain-relief pattern formed by an Ag bilayer on Pt(111) is a prominent example for dislocation networks and their use as nanotemplates. However, its atomic structure has not been solved. Combining scanning tunneling microscopy, low-energy ion scattering, and x-ray photoelectron diffraction, we demonstrate that, unexpectedly, about 22% of the atoms exchange across the Ag/Pt interface, and that the partial dislocations defining the trigonal network are buried in the Pt interface layer. We present an embedded-atom-method simulation identifying the lowest energy structure compatible with all experimental findings. Strain-relief patterns formed by heteroepitaxial films act as templates for the self-assembly of highly ordered nanostructure superlattices. 1 The first system where the growth kinetics has been studied is a superlattice of Ag islands grown on a trigonal network (TN) formed by two monolayers (ML) of Ag/Pt(111). 2,3 The system has been modeled by assuming a network of surface Shockley partial dislocations 4 acting as repulsive line defects and thus confining the deposited Ag adatoms into the (25 × 25) unit cells. 2,3 However, the topmost Ag layer of the TN has recently been shown to be nearly perfectly hexagonal; hence there are no partial dislocations at the surface. 5,6 Nevertheless, the TN exhibits remarkable template functions. Well-ordered sputter holes 5 and adsorbed organic molecules, 6-8 as well as strong spatial variations of the local work function 9 have been reported. A fundamental understanding of any template function evidently requires knowledge of its atomic structure. 10 Despite considerable efforts, a realistic atomistic model has not yet been developed for the 2 ML Ag/Pt(111) TN.Silver on Pt(111) is a system with a particularly rich surface phase diagram. Submonolayers deposited at room temperature (RT) are pseudomorphic for islands smaller than 20 nm, whereas larger islands form surface partial dislocations. 11,12 Counterintuitively, the dislocations disappear at a full monolayer (defined as one Ag atom per Pt substrate atom), where the stress resulting from the misfit is largest. This has been related to the chemical adatom potential and to the presence of an adatom gas. 11 Further growth at RT leads to a striped phase (SP) 4,13 with pairs of partial dislocations. Upon annealing to 800 K the SP transforms into the stable TN structure. 4 Deposition of Ag submonolayers at higher temperature, or their annealing at T > 620 K, leads to a real mixture formed by monolayer Ag clusters embedded into the uppermost atomic substrate layer for a coverage < 0.5 ML and the reverse for 0.5 ML < < 1.0 ML. 14 This mixing has been confirmed by CO titration 15 and He-atom scattering 16 and is confined to the uppermost layer; hence, the name "surface alloy". 17 For 1.0 ML, especially for = 2 ML, Ag and Pt were believed to be phase separated up to the Ag desorption temperature.In this paper we report the surprising observation that the system mixes again for 2 ML Ag in the TN structure. This mi...

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“…42,43 • A pure description in terms of interfacial energies might be insufficient since charge transfer processes between the adlayer and the substrate may induce a big interfacial stress. 44,45 This stress (not included in Eq. 1) can influence the growth mode of the adlayer significantly.…”

Section: Sample Preparationmentioning

confidence: 99%

Charge Transport Through Thin Amorphous Titanium and Tantalum Oxide Layers

Stella¹,

Kovacs²,

Diesing³

et al. 2011

J. Electrochem. Soc.

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Heterosystems of metal/insulator/gold type with titanium oxide and tantalum oxide as internal barriers are studied using internal photoemission (IPE), field induced current transport (current transients after voltage steps) and chemical reaction induced current transport (chemicurrent). IPE investigations over a broad energy range from 0.8 to 4.5 eV allow a determination of the interstitial layers band gap and the maximum height of the internal tunnel barrier. The built-in field of the heterosystem is derived by the evaluation of the slope in the photoyield versus photon energy plot. Current transients recorded after voltage steps allow the determination of the heterosystems time constants which generally have a value of some milli seconds. In titanium oxide systems additional time constants with values of several 100 s appear for bias voltages >0.5 V. These time constants are assigned to slow processes altering the height of the titanium oxide barriers. and electron sources in spin polarized tunneling. 5 In the present work we present a comparative investigation of charge transport induced by different methods through thin oxide films. Charges are driven through the oxide by: i) application of a device voltage ii) illumination of the samples with monochromatic light at variable photon energies, leading to spectra of internal photoemission, iii) non adiabatic chemical surface reactions. All three methods are combined since it is possible to derive the nature of excited carriers transport through heterosystems.6 For aluminum and tantalum oxide heterosystems it was found, that they form a high pass filter for excited electrons and holes (defect electrons). 4,6 Since the height of the internal barriers for electrons and holes can be modified by an applied bias voltage, it became possible to characterize the spectra of excited charge carriers with energies below the vacuum barrier. Bias voltages of up to 1 V could be applied to the system which had an internal barrier height of 3 eV.To increase the detection efficiency for excited electrons travelling from the surface of a metal towards the bulk, one can either decrease the thickness of the top metal film of a heterosystem or one can reduce the height of the internal barrier. But with a decrease of the internal barrier the method of applying a bias voltage for tuning this barrier might become problematic, since tunnel currents become larger with lower barrier heights. Additionally, the influence of midgap states, impurities and remanent changes of the internal barrier might become more relevant for lower barriers.8,9 These problems are adressed in the present work.A comparison of metal/insulator/metal heterojunctions is presented with potentiostatically formed titanium and tantalum oxide as interjacent insulating layer. Titanium and tantalum oxide were chosen since both have bandgaps smaller than 4.5 eV.10 For amorphous tantalum oxide values of 4.2 eV are typical 11,12 whereas crystalline samples show values of 3.9-4.5 eV.12,13 Bulk titanium oxide values are for rut...

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Giant Surface Stress in Heteroepitaxial Films: Invalidation of a Classical Rule in Epitaxy

Cited by 95 publications

References 22 publications

Surface stress in the self-assembly of alkanethiols on gold probed .by a force microscopy technique

Surface stress in the self-assembly of alkanethiols on gold probed .by a force microscopy technique

Interface-confined mixing and buried partial dislocations for Ag bilayer on Pt(111)

Charge Transport Through Thin Amorphous Titanium and Tantalum Oxide Layers

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