Solid Surfaces Analysis Group & Electron Microscopy Laboratory Nanophysics Michael Hietschold Solid Surfaces Analysis Group & Electron Microscopy Laboratory Institute of Physics Portland State University, May 2005

Content of the Whole Course 1st Lecture 1. Introduction 2. The Nanoscale in 2,1,0, and 3 Dimensions 3a. Surfaces and Interfaces – Geometrical Structure

Intermediate Lecture – SPM Nanoanalysis I. Nature of Resolution Limits – Near-Field Principle II. Scanning Tunneling Microscopy / Spectroscopy / Manipulation III. Scanning Force Microscopies IV. Other Near-Field Microscopies

2nd Lecture 3b. Surfaces and Interfaces – Electronic Structure 4. Semiconductor Heterostructures 3rd Lecture 5. 2-Dimensional Electron Gas 6. Quantum Interference, Molecular Devices, and Self-Assembling 7. Outlook

1.Introduction History: Richard Feynman 29th December 1959 (APS Meeting at Caltech): „There is plenty of room at the bottom“ Fiction : Molecular electronics (F.L.Carter 1982) Reality: Daily-life nanotechnology (e.g. ultrathin films, ultra-precision manufacturing, self-organized and -assembled structures, ...) Breakthrough: Scanning probe techniques Nanotechnology needs Nanoscience !!!

Dimensional Considerations 1 nm = 10-9 m = 0.001 µm 1/1000 extension of a malaria bacterium A few nearest-neighbor distances in solids Fe (bcc): d = 0.25 nm 1 nm

Role of surface effects increases with decreasing dimensions A / V = 6a2 / a3 = 6 / a = 6 V-1/3 V = a3 (2a)3 = 8 a3 (5a)3 = 125 a3 (10a)3 = 1000 a3 Percentage of „surface atoms“: 100% 100% 78,4% 48,8% Macroscopic: V = (108a)3 = 1024 a3 A = 6 (108a)2 = 6 1016 a2 Percentage of surface atoms: 6 10-8 % !!! (negligible)

Behavior of extensive physical quantities Classical macroscopic physics (thermodynamics): E = ε V = e N Geometry-dependent mesoscopic quantities: Sphere: E = ε V + εSurface(R) A = e N + eSurface(R) N2/3   Cube: E = ε V + εSurface A + εEdge L + εCorner 8 ≈ e N + eSurface N2/3 + eedge N1/3 + eCorner N0 εSurface = εSurface (∞)

Quantum Electrodynamics Application of Basic Physical Theories – Classical vs. Quantum Physics: Classical Mechanics Electrodynamics Thermodynamics Quantum Mechnics Quantum Electrodynamics Quantum Statistics   mesoscopic phenomena (quasiclassical regime)

Bottom-up and top-down approaches „chemical/syntheti-cal approach“ (scaling-up from the atomic entities) Top-down: classical approach of miniaturization (scaling down from the macroscopic world)

2. The Nanoscale 1, 2, 3 Dimensions Number of Nano-Dimensions: 1 – Nanofilms 2 – Nanowires 3 - Nanodots One can start by creating Nanofilms on a substrate and proceed to Nanowires and Nanodots by lateral lithography

Other Nanoobjects Nanocomposites Nanoporous Systems High-velocity deformed nanostrucutred Ni http://www.nanodynamics.com/ndMaterials.asp Nanoporous luminescent Si http://www.chem.ucsb.edu/~buratto_group/PorousSilicon_1.htm

Supramolecular Architectures J.-M.Lehn http://www.iupac.org/publications/pac/1994/pdf/6610x1961.pdf 3-dimensional functional structures according to the molecular geometri- cal and electronic structures C.J.Kuehl http://www.iupac.org/news/prize/2002/Kuehl-essay.pdf

3. Surfaces and Interfaces 3.1. Macroscopic Description Surface Energy: E = ε0 V < 2 [ ε0 (½ V) + εSurface A ]    εSurface > 0 Relaxation „frozen“ surface relaxes towards equilibrium Classical cleavage

Wulff‘s Construction Surface tension γ: γ = ∂ F / ∂ A ∫ γ(n) dA  Min.    ∫ γ(n) dA  Min. (whole surface) γ-plot: inner enveloppe of γ(n) determines crystal shape in equilibrium

Phase Boundaries Interfacial tensions Young‘s equation: γS = γS/F + γF cos Ф determines modes of thin film growth: Frank - van der Merwe (complete wetting) Vollmer-Weber (islands)

Frank-van der Merwe Stranski-Krastanov Vollmer-Weber Ф = 0; γS > γS/F + γF Ф > 0; γS < γS/F + γF Atomic interactions: Sub-Ads > Ads-Ads Ads-Ads > Sub-Ads only valid in equilibrium  supersaturation changes conditions

3.2. Structure and Crystallography of Surfaces TLK model (terraces, ledges, kinks) Burton, Frank, Cabrera 1935

Fundamental Surface Lattices 5 Bravais lattices in 2 dimensions belonging to 10 point groups in 3 dimensions: 14 Bravais lattices, 32 point groups

Miller Indices z Sections cutted from the axis Take inverse of them Multipy to get the smallest Integers Axes parallel to surface – index 0 y x (1-10) (211)

varying distances between lattice planes z Surface Relaxation varying distances between lattice planes (metals) Surface Recon-struction Change of (lateral) atomic arrangement on the surface (semiconductors)

Adsorbate Structures z

Description of superstructures: R = m a1 + n a2   Adsorbate / Rec. Surface Lattice: b1 = m11 a1 + m12 a2 b = M a b2 = m21 a1 + m22 a2 Area of new unit cell: |b1 x b2| = det M |a1 x a2| integer simple det M rational  coincidence superlattice irrational incommensurate

Exemples: Real space diffraction image

Structure of interfaces z crystallinity and sharpness characterize solid-solid interfaces