Surface morphology of laser-superheated Pb(111) and Pb(100)

Zhang, Z. H.; Lin, Bor-Luh; Zeng, Xinglin; Elsayed-Ali, Hani E.

doi:10.1103/physrevb.57.9262

Cited by 12 publications

(11 citation statements)

References 29 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…There exist experimental studies that show superheating of solids, but in these experiments the crystals are either confined in a nonmelting matrix 3,4 or the experiments reveal superheating of one particular crystal surface only. 5,6 The melting of a single-chain polymer system is expected to be different. The reason is that all the polymer units are restricted by the strong covalent bonds along the chain.…”

mentioning

confidence: 99%

Free energy barrier to melting of single-chain polymer crystallite

Frenkel

Mathot

2003

The Journal of Chemical Physics

View full text Add to dashboard Cite

We report Monte Carlo simulations of the melting of a single-polymer crystallite. We find that, unlike most atomic and molecular crystals, such crystallites can be heated appreciably above their melting temperature before they transform to the disordered ''coil'' state. The surface of the superheated crystallite is found to be disordered. The thickness of the disordered layer increases with super-heating. However, the order-disorder transition is not gradual but sudden. Free-energy calculations reveal the presence of a large free-energy barrier to melting. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1553980͔It is easy to supercool liquids but difficult to superheat solids. The reason is that the free surface of a crystallite can melt at a temperature that is well below the bulk melting temperature.1,2 As a consequence, solids usually melt from the surface inward without significant superheating. There exist experimental studies that show superheating of solids, but in these experiments the crystals are either confined in a nonmelting matrix 3,4 or the experiments reveal superheating of one particular crystal surface only. 5,6The melting of a single-chain polymer system is expected to be different. The reason is that all the polymer units are restricted by the strong covalent bonds along the chain. This implies that, when a single polymer partially melts ͑or dissolves͒, the molten units cannot escape from the surroundings of the crystallite. Rather, they stay around as a ''corona'' and can, in this way, affect the remainder of the melting process. The simulations presented below show that these features make the melting of polymer crystallites qualitatively different from that of atomic or molecular crystals.Lattice models provide a highly simplified picture of freezing and melting. Nevertheless, it has been shown that such models are sufficiently flexible to account for the phenomenon of surface melting in simple ''atomic'' systems. 7 We used a polymer lattice model described in Ref. 8 to study the melting of a single-chain crystallite. In this model, polymers live on a simple cubic lattice, but the monomermonomer bonds on the chain can be directed both along main axes of the lattice and along the face and body diagonals: 6ϩ12ϩ8ϭ26 directions in all. The polymers can be semi-flexible and have attractive nearest-neighbor interactions. For each bond-bond connection along the chain, all noncollinear bonds are assumed to have the same energetic cost defined as E c . The attractive interactions can be anisotropic: Parallel polymer bonds attract more strongly than nonparallel bonds ͑their difference is defined as E p ), or isotropic: the site-site energy change ͑defined as B͒ when forming one polymer-solvent contact from polymer-polymer and solvent-solvent contacts. By varying E c , B and E p , we can ''tune'' the ''phase-diagram'' of a single polymer. A flexible (E c ϭ0) polymer with a large B but small E p undergoes a coil-globule transition.9 In contrast, a flexible polymer with a large E p but small B wi...

show abstract

mentioning

confidence: 99%

Free energy barrier to melting of single-chain polymer crystallite

Frenkel

Mathot

2003

The Journal of Chemical Physics

View full text Add to dashboard Cite

show abstract

“…In our experiment, we did not observe such step collapse below 590 K; however, by using laser heating we observed a sudden collapse of steps after laser melting. 8 This suggests that the temperature for facet formation in our experiments should be higher than 590 K. The theoretically estimated temperature, 583 K in MD simulations, seems to be small. 20 In HRLEED experiments, a sudden collapse of steps above 580 K was observed.…”

Section: Resultsmentioning

confidence: 60%

“…Based on kinematic diffraction, we conclude that these FWHM changes in the RHEED profiles are completely due to changes in the average terrace width and string length, not vacancies. 8,10,15,21 Without taking into account the instrumental broadening, the RHEED intensity profile from a twodimensional monatomic stepped surface has a sharp peak, due to the flat surface, and a diffuse intensity in the shape of a Lorentzian function with FWHM depending on step density. 10,15 For a vicinal stepped surface, the diffuse intensity is more complicated and gives a splitting shape.…”

Section: Resultsmentioning

confidence: 99%

Temperature dependence of step density on vicinal Pb(111)

Zhang

Elsayed-Ali

1998

Phys. Rev. B

Self Cite

View full text Add to dashboard Cite

The temperature dependence of step density on the vicinal Pb͑111͒ surface is investigated using reflection high-energy electron diffraction. When the temperature is increased from 323 to 590 K, the average terrace width and the average string length at the step edge decrease from 85Ϯ25 to 37Ϯ16 Å and from 220Ϯ33 to 25Ϯ8 Å, respectively. Thermal step collapse on the Pb͑111͒ surface near its bulk melting temperature is not observed. Above 530Ϯ7 K, the change in the string length at the step edge with temperature becomes small, and the intensity of the ͑00͒ beam is significantly decreased. We conclude that partial step melting at the step edge occurs at the surface above 530Ϯ7 K.

show abstract

“…1. 21 For the GaAs͑100͒ surface studied, the measured average string length is 135Ϯ23 Å. The sample is then exposed to partially dissociated hydrogen at 1ϫ10 Ϫ6 Torr for 3 h, while kept at 500°C.…”

Section: Resultsmentioning

confidence: 99%

“…21 Figure 1 shows the RHEED intensity Rϭ(I p ϪI b )/I b and QE after heating at temperatures up to 600°C, represented by open circles and squares, respectively. 21 Figure 1 shows the RHEED intensity Rϭ(I p ϪI b )/I b and QE after heating at temperatures up to 600°C, represented by open circles and squares, respectively.…”

Section: Resultsmentioning

confidence: 99%

Atomic hydrogen-cleaned GaAs(100) negative electron affinity photocathode: Surface studies with reflection high-energy electron diffraction and quantum efficiency

Elamrawi

Hafez

Elsayed-Ali

2000

Journal of Vacuum Science &Amp; Technology A: Vacuum, Surfaces, and Films

Self Cite

View full text Add to dashboard Cite

Role of oxygen in semiconductor negative electron affinity photocathodesAtomic hydrogen cleaning of InP(100): Electron yield and surface morphology of negative electron affinity activated surfacesThe quantum efficiency of a vicinal GaAs͑100͒ negative electron affinity ͑NEA͒ photocathode is studied and correlated to the surface morphology. Cleaning of a GaAs͑100͒ vicinal surface by atomic hydrogen and by heating are investigated using reflection high-energy electron diffraction ͑RHEED͒. After atomic hydrogen cleaning at 500°C, the GaAs surface exhibits a streaky ͑2ϫ4͒-reconstructed RHEED pattern. When the GaAs͑100͒ surface is activated to NEA by the alternate deposition of cesium and oxygen, a quantum efficiency of ϳ9% is measured. The photocathode quantum efficiency correlates with the out-of-phase RHEED intensity measured before activation. After the quantum efficiency decreases with operating time, further atomic hydrogen exposure also produces a ͑2ϫ4͒ pattern. Surfaces prepared or revived by atomic hydrogen produce brighter out-of-phase electron diffraction patterns and, when activated to NEA, higher quantum efficiency compared to those that are heat cleaned.

show abstract

Surface morphology of laser-superheated Pb(111) and Pb(100)

Cited by 12 publications

References 29 publications

Free energy barrier to melting of single-chain polymer crystallite

Free energy barrier to melting of single-chain polymer crystallite

Temperature dependence of step density on vicinal Pb(111)

Atomic hydrogen-cleaned GaAs(100) negative electron affinity photocathode: Surface studies with reflection high-energy electron diffraction and quantum efficiency

Contact Info

Product

Resources

About