Nanostrukturphysik (Nanostructure Physics)

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Nanostrukturphysik (Nanostructure Physics) Prof. Yong Lei & Dr.

de/3dnanostrukturierung/ Vorlesung: Wedsnesday 9:00 10:30, C 108 Übung: Friday (G),

1 Nanostrukturphysik (Nanostructure Physics) Prof. Yong Lei & Dr. Yang Xu Fachgebiet 3D-Nanostrukturierung, Institut für Physik Contact: Office: Unterpoerlitzer Straße 38 (Heisenbergbau) (tel: 3748) Vorlesung: Wedsnesday 9:00 10:30, C 108 Übung: Friday (G), 9:00 10:30, C 110 (a) (b 1 ) (b 2 ) UTAM-prepared free-standing one-dimensional surface nanostructures on Si substrates: Ni nanowire arrays (a) and carbon nanotube arrays (b).

2 Class 1: a general introduction of fundamentals of nanostructured materials, and definition Class 2: research at 3D-Nanostructuring (01) Class 3: research at 3D-Nanostructuring (02) Class 4: graphene Class 5: 2D atomically thin nanosheets Class 6: optical properties of 1D nanostructures Class 7: carbon nanotubes Class 8: lithium-ion batteries: Si nanostructures Class 9: solar water splitting I: fundamentals Class 10: solar water splitting II: nanostructures for water splitting Class 11: solar cells

3 Contents Optical properties of 1-D nanostructures

4 Large-scale free-standing metallic nanowires for 3D surface patterns: (Left): top view of nanowire array of an area of about 775 μm 2. (Right): high regularity of nanowire arrays.

5 Features of optical properties of 1-D nanostructures Sharp and discrete features in absorption spectra & band-edge PL (photoluminescence) shift and enhancement results of quantum confinement effects. Anisotropic PL, highly polarized along axial direction the large dielectric contrast between nanowire and surrounding environment. Efficient migration of electrons and holes to surface of nano-structures allows them to participate in chemical reactions before recombining enhance the efficiency of solar cells.

6 Features of optical properties of 1-D nanostructures Nanowires with optical properties tuned by changing aspect ratio. Single crystalline and well faceted nanowires can function as effective resonance cavities; lasing properties. Oxygen defects and chemical surface absorption plays an important role to photo-sensitivity and PL of oxide 1-D nanostructures.

1st finding (obvious evidence) of the quantum confinement effect of 1-D nanostructures: Holmes JD, Science, 2000.

7 1st finding (obvious evidence) of the quantum confinement effect of 1-D nanostructures: Holmes JD, Science, Defect-free Si nanowires with uniform diameters range from 4 to 5 nm and length of several micrometers- by a supercritical solution-phase approach. Si nanowires synthesized at 500 o C at pressures of 200 bar (A and B) and 270 bar (C and D). The nanowires are highly crystalline. In both (B) and (D), the lattice fringes are Si (111) planes, separated by 3.14 Ǻ.

8 The absorption edge of the Si nanowires was strongly blue-shifted from the bulk indirect band gap of 1.1 ev and showed sharp discrete absorption features and strong band-edge PL, results from quantum confinement effects. The 100> oriented wires have a much higher exciton energy than that of the 110> oriented wires.

9 Band-Gap Variation of Size- and Shape-Controlled CdSe Quantum Rods (Li LS, Nano Letters, 2001) TEM images of 4 CdSe nanorod samples. The scale bar is 50 nm. PL spectra of 3.7 nm wide CdSe rods with lengths: 9.2, 11.5, 28.0, 37.2 nm, respectively (from left to right), excited at 450 nm.

10 Band gap of CdSe quantum rods vs length and width viewed from two different angles. The data are fit in 1/length (1/L), 1/width (1/W), and aspect ratio (L/W). The best fit obtained is: /L /W (L/W) (L/W) /L /W 3.

Advanced Materials, 2003) 10 periods of Zn 0.8 Mg 0.

11 Quantum confinement observed in ZnO/ZnMgO multiple quantum well nanorod heterostructures (Park WI, Advanced Materials, 2003) 10 periods of Zn 0.8 Mg 0.2 O/ZnO on ZnO nanorods. A: 1.1 nm wells and B: 2.5 nm wells.

12 Quantum confinement in 1-D nanostructures Quantum confinement can be approximately described by a simple particle-in-a-box type mode: ΔE = 1/d n (d: diameter, 1 n 2) size dependence of bandgap. The quantum confinement effect in semiconductor nanodots and nanowires did not exactly follow the particle-in-a-box prediction.

13 Quantum confinement in 1-D nanostructures A particle-in-a-cylinder mode: The calculated energy shift ΔE, relative to the bulk band gap as a function of the nanowire radius R, is given by: L is the length of the cylinder, m* is the reduced effective exciton mass (m e m h /(m e + m h )), ħ is Planck s constant, e is the electron charge. The first term represents the size-dependent kinetic energy confinement by the walls of the nanowire cylinder. The second term is the attractive Coulomb interaction between electron and hole. This mode provides excellent fits to experimental results.

14 Nanowire Lasing (Yang PD s group) Nanowires with flat facets at both end can be used as optical resonance cavities to generate coherent light. UV lasing at RT has been demonstrated for ZnO and GaN nanowires with epitaxial arrays and single nanowires. ZnO and GaN are wide bandgap semiconductors (3.37, 3.42 ev) suitable for UV-blue optoelectronics. The large binding energy for excitons in ZnO ( 60 mev) permits lasing via exciton-exciton recombination at low excitation conditions.

15 Well-faceted nanowires with diameters from 100 to 500 nm support axial Fabry-Perot waveguide mode: Δλ = λ 2 /[2Ln(λ)] where L is the cavity length and n(λ) is the group index of refraction.

16 Diffraction prevents smaller wires from lasing. ZnO and GaN nanowires produced by VLS growth are cavities with low intrinsic finesse (F) owing to the low reflectivity (R) of their end facets ( 19%): F = πr1/2/(1-r)] the confinement time for photons is short and photons travel an average of 1 to 3 half-passes before escaping from the cavity.

17 Nanowire Lasing The transition from spontaneous PL to lasing is achieved by exciting high density of wires via pulsed UV illumination (pumping). 3 regions: (a) spontaneous emission, (b) stimulated emission (lasing starting) above a certain threshold, (c) saturation (lasing) at high pump power. The lasing thresholds vary several orders of magnitude as a consequence of different nanowire dimensions, the lowest threshold observed for ZnO is 70 nj cm -2 and for GaN 500 nj cm -2. (a) Spectra of light emission from GaN/AlGaN core-shell nanowires below, near and above lasing threshold (about 2 3 µj/cm2). (b) The power dependence of output integrated emission intensity.

18 Lasing emission are localized at the ends of nanowires, suggests strong waveguide behavior - consistent with axial Fabry-Perot mode. spontaneous emission stimulated emission (lasing)

The ultrafast dynamics of the lasing in ZnO nanowires and nanobelts Transient PL spectroscopy is used to detect carrier relaxation dynamics near the lasing threshold.

19 The ultrafast dynamics of the lasing in ZnO nanowires and nanobelts Transient PL spectroscopy is used to detect carrier relaxation dynamics near the lasing threshold. (a) SEM of as-grown nanowire array and (b) single nanowire dispersed on sapphire substrate. Inset: far-field image of nanowire emission. (c) SEM of as-grown nanobelts and (d) single dispersed belt on silicon. Inset: far-field image of belt lasing emission.

20 The ultrafast dynamics of the lasing in ZnO nanowires and nanobelts Above the lasing threshold, a fast decay of PL was observed, with a fast component (< 10 ps) corresponding to exciton-exciton lasing and a slow component ( 70 ps) owing to free-exciton spontaneous emission. (a) PL/lasing spectra of single ZnO nanowire near the lasing threshold (excitation 1 µj/cm2) and (b) transient PL response. Long decay component is about 70 ps and short component is about 9 ps (red) and 4 ps (black).

21 Nanowire Lasing The useful applications for nanowire lasers require that they are integrated in circuits and activated by electron-injection rather than optical pumping. Lieber and coworkers have made progress in this direction by assembling n-type CdS nanowire (Fabry-Perot cavities) on p- Si wafers to form the required heterojunction for electricaldriven lasing: Single-nanowire electrically driven lasers (Lieber et al., Nature 2003)

First, optical-pumped Single-nanowire lasers a, A nanowire as an optical waveguide, with cleaved ends of a Fabry Perot cavity. b, A cleaved CdS nanowire end.

22 First, optical-pumped Single-nanowire lasers a, A nanowire as an optical waveguide, with cleaved ends of a Fabry Perot cavity. b, A cleaved CdS nanowire end. c, RT PL image of a CdS nanowire excited (low-left corner, power 10 mw) about 15 mm away from the nanowire end. The white arrow highlight the nanowire end. d, PL spectra obtained from the body of the nanowire (blue) and the end of the nanowire (green) at low pump power (10 mw). e, Spectrum from the nanowire end at higher pump power (80 mw) showing periodic intensity variation.

23 Optically pumped nanowire laser: Emission spectra from a CdS nanowire end with a pump power of 190, 197 and 200 mw (red, blue and green) recorded at 8 K.

Single-nanowire electrical-driven lasers To investigate nanowire injection lasers, a hybrid structure was used: n-type CdS nanowire laser cavities are assembled onto p-si electrodes.

24 Single-nanowire electrical-driven lasers To investigate nanowire injection lasers, a hybrid structure was used: n-type CdS nanowire laser cavities are assembled onto p-si electrodes. An image of a typical device is shown in b. Images of the RT electroluminescence produced in forward bias from these hybrid structures (b) exhibit strong emission from the exposed CdS nanowire ends.

6 nm, come from both spontaneous and stimulated emission.

25 (d) At low injection currents, the end emission shows a broad peak (spontaneous emission). Above 200 ma threshold, the spectrum quickly collapsed into a few very sharp peaks with a dominant emission at nm, come from both spontaneous and stimulated emission. (e) Low-T measurements on independent CdS injection laser devices show spontaneous emission spectrum, it can collapse to a sharp peak of lasing, the results are very similar to the low-t optically pumped results.

26 Techniques for device fabrication of semiconductor lasers is costly and difficult to integrate directly with Si microelectronics. There are considerable interests in using organic molecules, polymers, and inorganic nanostructures for lasers, because these materials can be integrated into devices by chemical processing. And stimulated emission or lasing have been reported for optically pumped inorganic nanowires and organic systems. Electrical-driven nanowire lasers might be assembled in arrays capable of emitting a wide range of colors, used in flat displays.

27 Field-emission Display (FED) Much attentions to explore using of semiconductor 1-D nanostructures as field-emitters: low work functions, high aspect ratios, high mechanical stabilities, high electrical and thermal conductivities. Field-emission is one of the main features of nanostructures, and is of great commercial interest in FEDs and other electronic devices. Progresses in the synthesis and assembly of nanostructures has resulted in a considerable increase in the current density and lowering of turn-on voltage. Besides CNTs, some other inorganic semiconductor nanostructures used for fieldemitters, such as ZnO, Si, WO 3, SiC, ZnS, AlN.

28 Field-emission is a quantum tunneling process: electrons pass from an emitting material (negatively biased) to the anode through a barrier (vacuum) with a high electric field. Highly dependent both on properties of material and shape of cathode, materials with higher aspect ratios and sharper edges (nanowires or nanotubes) produce higher field-emission currents. The current density J produced by an electric field E (the Fowler Nordheim equation): J = (Aβ 2 E 2 /ø) exp(-bø 3/2 /βe), or ln(j/e 2 ) = ln(aβ 2 /ø) Bø 3/2 /βe, (1) I = S J, E = V/d, (2) (A: A ev V -2, B: ev -3/2 V μm -1, S: emitting area, V: applied potential, I: emission current, β: field enhancement factor, d: distance between sample and anode, ø: work function) β is related to emitter geometry and crystal structure, and spatial distribution of emitting centers: β = h/r, (h is the height and r is radius of curvature of emitter). Materials with elongated geometry and sharp tips or edges can greatly increase an emission current.

The emission current is strongly dependent on three factors: (i) work function of an emitter surface, (ii) radius of curvature of the emitter end, (iii) emission area.

29 The emission current is strongly dependent on three factors: (i) work function of an emitter surface, (ii) radius of curvature of the emitter end, (iii) emission area. A lower work function material can produce a higher electron emission current. However, not all low work function materials are ideal for constructing field-emission cathodes (a) The emission occurs from tip of an emitter. (b) The emitter can have different emission currents depending upon the tip geometry, such as (i) round tip, (ii) blunt tip and (iii) conical tip. (W. Z. Wang, et al., Adv. Mater., 2006) For a given material, emission current can be enhanced by increasing its aspect ratio, assembling it into arrays, or decorating its surface with a lower work function material.

ZnO 1-D nanostructures (belts and wires) Aligned ultra-long ZnO nanobelts.

(b) beltlike structures with a width up to 6 μm. (c) TEM image of a single belt.

reflects much smaller thickness of belts compared to widths.

30 ZnO 1-D nanostructures (belts and wires) Aligned ultra-long ZnO nanobelts. (a) optical photo of nanobelts with length of several mm. (b) beltlike structures with a width up to 6 μm. (c) TEM image of a single belt. Its transparency to electron beam (can see a copper TEM grid beneath belt) clearly reflects much smaller thickness of belts compared to widths. The HRTEM image of this belt is shown in (d) shows the perfect crystallinity and defect-free nature of nanobelts. (Wang WZ, Adv Mater, 2006)

31 Field-emission performances of aligned ultralong ZnO nanobelts. (a) field emission current density applied field (J F) curve. The turn-on electric field is about 1.3 V μm -1. (b) Fowler Nordheim plot of nanobelts, fits well to the linear relationship given by Fowler Nordheim equation: ln(j/e 2 ) = ln(aβ 2 /ø) Bø 3/2 /βe From the slope of fitted straight line in (b), the ZnO nanobelts have a very high fieldenhancement factor of , which is the result of the extremely high aspect ratio of the emitter geometry.

$Within a bundle they are aligned. In many cases even perfectly parallel ensembles are visible. The electron diffraction patterns show the similar orientation of nanobelts along the [001] direction.$

32 ZnS nanobelt arrays ZnS nanobelt arrays. Length of belts is about several hundreds of micrometers; some of them may even be as long as a millimeter. These nanobelts form in bundles. Within a bundle they are aligned. In many cases even perfectly parallel ensembles are visible. The electron diffraction patterns show the similar orientation of nanobelts along the [001] direction. HRTEM images of an individual belt display the defect-free (001) lattice plane of wurtzite Zn and confirm the [001] growth direction.

33 The orientation-ordered ZnS nanobelt arrays have much improved fieldemission properties as compared to random nanowires: a low turn-on field (~ 3.55 V μm -1 ) and a high field-enhancement factor (~ 1850).

Some effective routes were developed to enhance the field-emission performances: e.g.

(~ 1 nm diameter) and ultra-high density (10 9 3 10 11 cm -2 ) SiCcapped Si nanotip arrays.

1000; (b) the high density nature of nanotip arrays. (c) TEM image of a SiC-capped Si nanotip.

34 Some effective routes were developed to enhance the field-emission performances: e.g. only ~ 1 V μm -1 turn-on field at a 3 ma cm -2 current density was achieved from ultra-sharp (~ 1 nm diameter) and ultra-high density ( cm -2 ) SiCcapped Si nanotip arrays. SiC-capped silicon nanotips: (a) nanotips of about 1 μm height with an aspect ratio of about 1000; (b) the high density nature of nanotip arrays. (c) TEM image of a SiC-capped Si nanotip. The inset is a magnified lattice image at the interface between the Si and SiC. (Lo HC, Appl Phys Lett, 2003)

35 A typical field emission data obtained from SiC-capped silicon nanotips demonstrating ultralow turn-on electric fields (only ~ 1 V μm -1 turn-on field at a 3 ma cm -2 current density).

Metallic strips were prepared on both sides of ceramic plate (perpendicular to each other while electrically insulated by ceramic).

36 WO 3 1-D nanostructures (Chen J, Appl Phys Lett, 2007) FED: WO 3 nanowires as cathode, with a cathode plate (consists of nano-emitters on a substrate). Anode: phosphor screen. Gate plate: a ceramic plate with round apertures. Metallic strips were prepared on both sides of ceramic plate (perpendicular to each other while electrically insulated by ceramic). 8 8 arrays of WO 3 nanowires on a Si wafer (a). Diameter of each cathode is ~ 1 mm, distance between pixels is 2.5 mm. The dark spots on anode correspond to the pixels (b).

37 The functioning of the device, where Arabic and Chinese characters appear by switching of individual spots. Each pixel could be accurately addressed without interference.

38 ZnO 1-D nanostructures and nano-generator Among the known 1D nanomaterials, ZnO has three key advantages: It has both semiconductor and piezoelectric properties; It is relatively bio-safe and bio-compatible, and can be used for biomedical applications with little toxicity;

39 Nanogenerator based on piezoelectric behavior of ZnO nanowires Implantable biomedical electronic device (pacemakers), is fast increasing in the past two decades. A major shortcoming: all implantable biomedical devices need battery replacement. Surgeries to replace battery 15K EUR + possible danger. Highly desirable for implanted biomedical devices to be self-powered, harvest electrical energy from natural energies in human body. Fabrication of nanoscale generators as power supplies for implantable biomedical devices: a self-powered implantable biomedical device, avoid the medical surgery to replace batteries reduce the size of integrated system of a device and its power source. The realization of a highly efficient nano-generator with sufficient energy output to power a biomedical device presents an important issue to the fields of biomedical technology as well as nano-science.

Mechanism of piezoelectric discharging of ZnO NW with AFM scanning (Z. L. Wang et. al., Science 312, 242 (2006) When a ZnO NW is bent by a Pt-coated AFM tip, a strain is produced.

40 Mechanism of piezoelectric discharging of ZnO NW with AFM scanning (Z. L. Wang et. al., Science 312, 242 (2006) When a ZnO NW is bent by a Pt-coated AFM tip, a strain is produced. Stretched side has positive potential and compressed side has negative potential. Schottky diode is formed (Pt/ZnO). Two processes: When tip contacts and bends NWs, interface of tip and stretched side is a reversely biased Schottky diode (ΔV=V m V S+ <0). Piezoelectric potential is created in NW, no charge flowing across Schottky diode although there is a piezoelectric potential in NW side (charge creation and accumulation process). When tip reaches compressed side of NW, a forward biased Schottky diode formed at interface (ΔV=V m V S- >0), external electrons can flow across interface under driving of piezoelectric potential, resulting in a discharging. (current output process).

Metallic membrane Nanowire array Side view (schematic) of assembled

top of nanowires extended into holes of a Pt nano-porous membrane.

41 Metallic membrane Nanowire array Side view (schematic) of assembled structure of proposed nano-generator: ordered ZnO nanowire arrays with top of nanowires extended into holes of a Pt nano-porous membrane. Insets show a template-prepared Ni membrane and Ni nanowire arrays. Lei Y., Jiao Z., Wu M. H., Wilde G., Ordered Arrays of Nanostructures and Applications in High-Efficient Nano-Generators, Advanced Engineering Materials, 9, 343, 2007.

42 (a) (b) (c) No current when NW is not deflected (a). Piezoelectric discharging is generated when NW is deflected by liquid flow and touch Pt, no matter what direction of liquid flow [(b) and (c)]. Arrayed NWs will be deflected at almost same time and in same way discharges of different NWs are collected at same time, realizing a stable and large DC output power a real device. When nano-generator is implanted into human body, different mechanical (body movement, muscle stretching, and blood pressure) and hydraulic (flow of body fluid and contraction of blood vessel) pressures on liquid sac will lead to a continuous wavy motion of water inside sac, forcing the ZnO NWs to contact Pt pore-walls continuously, thus resulting in a continuous piezoelectric discharging of each NW. Estimated output piezoelectric power of 1 NW ~ W. Output power from about NWs in 1 cm 2 area of nano-generator ~ W. This 5mW nanogenerator is sufficient to directly power a low-energy device like a pacemaker.

43 Direct-Current Nanogenerator (from Z. L. Wang et. al., Science 316, 102 (2007))

44 Optical applications of the metallic 1-D nanostructures - Nanometer-sized Metallic Barcodes Multimetal nanorods encoded with nanometer-sized stripes can be prepared. Complex striping patterns are prepared by sequential electrochemical deposition of metal ions into templates with uniformly sized pores (PAMs). The different reflectivity of adjacent stripes enables identification of the striping patterns by conventional light microscopy. This readout mechanism does not interfere with the use of fluorescence for detection of analytes bound to rods, as demonstrated by DNA and protein bioassays bioanalysis and biodetection (Nicewarner-Pena, et al., Science, 2001)

45 Synthesis of barcoded nanorods

Top: High contrast was observed between Ag (brighter sections) and Au (dark middle section) with 430-nm illumination.

46 Optical properties of barcoded 1-D nanorods. (A) SEM image (left) and optical microscope image (right) of the same Au-Ag-Au rods. (B) Wavelength dependence of reflectivity for bulk metals. (C) optical microscopy image of an Ag-Au-Ag barcode rod. Top: High contrast was observed between Ag (brighter sections) and Au (dark middle section) with 430-nm illumination. Bottom: No contrast using 600- nm excitation. (D) optical images and line profiles for a rod of composition Au-Ag-Ni-Pd- Pt with illumination at 430, 520, and 600 nm.

47 Optical (A) and SEM (B) images of an Au-Ag multi-stripe rod with about 550- nm Au stripes and Ag stripes of 240, 170, 110, and 60 nm (top to bottom). The same rod is shown in both images. Thus, it should be possible to distinguish large numbers of barcode patterns.

(iii) and (iv) show the fluorescence readout and the rod ID, respectively.

48 Bioassays performed on barcoded rods using fluorescence detection. (A) Sandwich DNA hybridization assay: (i) reveals the fluorescence readout; (ii) shows the rod ID The analyte, b, was omitted in the control sample. (iii) and (iv) show the fluorescence readout and the rod ID, respectively. (B) A simultaneous sandwich immuno-assay performed on barcode rods: (i) shows the reflectance optical microscopy image, which gives the barcode rod ID; (ii) and (iii) show the fluorescence readout with FITC and Texas Red filter sets, respectively. Immuno-assays on two different barcoded rods (1: 4μm-long Au-Ag-Au, 2 8μm-long Au-Ni-Au). Rods of type 1 were derivatized with capture antibody to human immunoglobulin G (IgG), rods of type 2 were derivatized with capture antibody to rabbit IgG. The samples were mixed and exposed to secondary antibodies. Each secondary antibody was labelled with fluorophores of different colors (green for antibody to human IgG, red for antibody to rabbit IgG). Note that the green fluorescence emanates mainly from rods of type 1, and red mainly from type 2. This indicates that the specific capture chemistries on the two classes of rods were able to selectively bind their target analytes.

49 Class 1: a general introduction of fundamentals of nanostructured materials, and definition Class 2: research at 3D-Nanostructuring (01) Class 3: research at 3D-Nanostructuring (02) Class 4: graphene Class 5: 2D atomically thin nanosheets Class 6: optical properties of 1D nanostructures Class 7: carbon nanotubes Class 8: lithium-ion batteries: Si nanostructures Class 9: solar water splitting I: fundamentals Class 10: solar water splitting II: nanostructures for water splitting Class 11: solar cells

50 Thank you and have a nice day!

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