Single Nano-objects Research Articles

Study of atomic force microscopy nanoindentation for the development of nanostructures

Abstract We have studied the fabrication of atomic force microscope (AFM) based nanotemplates using electrically controlled indentation (ECI) and a composite barrier (photoresist/alumina) that is resistant to the lithography process and presents good mechanical properties for indentation. The indentation process is affected by several factors such as the indentation speed, the trigger voltage and the barrier type. We have used the nanotemplate technique to fabricate small gold–gold nanocontacts (1–10 nm). In this limit, the size of the contacts that is obtained through the indentation process seems to be stochastic. However, low dimension, clean metallic contacts were achieved with high temporal stability and compatible with low temperature measurements. The fabricated nanotemplates are versatile and can be used in a wide range of applications, from nanojunctions to connecting a single nano-object. Small area metallic contacts can be used to study spin injection or ballistic transport.

Electrical and optical properties of a single Sb2Se3 nanorod

AbstractIn this paper we present the methods to chemically synthesize the Sb2Se3 nanorods and study the electrical and optical properties of a single Sb2Se3 nanorod with an average size of 70 nm in diameter and a length of about 1 μm to 2 μm. The techniques we devised can immobilize and allocate a single nano-object on an electron beam (E-beam) patterned smart substrate. It can also overcome the limitation of the spatial resolution of conventional optical techniques (∼ 1 mm) to obtain optical spectroscopy in an individual nano-object less than 100 nm in size. We also demonstrate the techniques in using E-beam defined metallic coordination markers as electrodes to measure the conductance of a single Sb2Se3 nanorod.

Theory of resonant radiation force exerted on nanostructures by optical excitation of their quantum states: From microscopic to macroscopic descriptions

We establish a theoretical framework that provides a bridge between the microscopic to macroscopic descriptions of the radiation force (RF) under a quantum resonance condition. By using this framework, we derive an explicit analytical expression to clearly demonstrate the properties of the resonant RF on nanostructures and related novel phenomena. (i) For a single nano-object the RF drastically changes with the size, shape, and quality of a nano-object due to the spatial correlations of the internal radiation field and the matter-excited states. This property is highly advantageous in the selective manipulation of quantum properties of nano-objects. (ii) For multiple nano-objects an attractive (repulsive) interobject radiation force (IRF) arises between nano-objects under the optical excitation of a particular coupled state of their spatially separated polaritons, and we term this state as ``polaritonic molecule.'' This IRF can be enhanced even between the nano-objects that have a large spatial separation if there exist intermediate nano-objects even with very weak induced polarizations, and this effect is termed as ``superinterobject radiation force.'' In addition, we clarify that a ``negative dissipative force'' arises when the electronic polarization in a particular nano-object is inverted by a photomediated interaction. Since the resonant RF and IRF depend on many degrees of freedom of both nanostructures and light, they will provide a great variety of optical control methods for the collective dynamics of nanocomposite materials.

Natural vibrations of nanotubes

431 Nanoobjects exhibit anomalous properties that are very attractive for applications and generally do not correlate with the properties of the macroscopic samples [1‐4]. For this reason, one of the key problems of nanomechanics is the determination of the mechanical and physical characteristics of nanoobjects. One of the most efficient methods for determining elastic moduli used in the mechanics of macroobjects is based on measurement of natural frequencies. A method for determining the natural frequencies of some nanostructures (nanotubes and nanocrystals) was proposed in [5, 6] on the basis of the measurement of natural frequencies of an “extended system” consisting of a highly oriented array (lattice) of identical nanotubes or nanocrystals, which are grown on a macroscopic substrate and are perpendicular to the substrate, and a single substrate. According to [5, 6], the spectrum of the natural frequencies of the large system can be separated into two components. One component of the system spectrum corresponds to the natural frequencies of single nanoobjects. The substrate remains almost static upon vibrations with these frequencies. The other component of the system spectrum is the spectrum of natural frequencies close to the natural frequencies of the substrate without nanoobjects. At these frequencies, the amplitude of the vibrations of the nanoobjects is much smaller than the amplitude of the substrate vibrations. Two modifications of the experimental procedure for determining the natural frequencies can be proposed in developing the method proposed in [5, 6]. Modification 1. Measure the natural frequencies of the nanotube lattice‐substrate or nanocrystal lattice‐ substrate. Measure the natural frequencies of the same substrate without nanoobjects. Compare the two spectra obtained. The frequencies in the system spectrum that have no correspondence among the frequencies in the substrate spectrum are the frequencies of the nanoobjects. Modification 2. Measure the natural frequencies of the system by detecting both the electromagnetic radiation of the nanoobjects, many of which are piezoelectric materials, and the substrate vibration amplitude. The resonant frequencies at which the substrate vibration amplitude is equal to zero are the natural frequencies of the nanoobjects.

Fabrication of planar cobalt electrodes separated by a sub-10 nm gap using high resolution electron beam lithography with negative PMMA

We present a fabrication process of cobalt nanoelectrodes compatible with spin-dependent transport measurements through a few or a single nano-object. It consists in etching a cobalt thin layer into pairs of planar nanoelectrodes separated by a nanometric gap using a negative Poly-MethylMethAcrylate (PMMA) mask patterned by high resolution electron beam lithography (HREBL). The irradiation parameters of 200 keV HREBL on PMMA have been investigated using atomic force microscopy (AFM) to define accurately the PMMA transformation from positive to negative tone. The influence of the electron dose and the designed gap on the final gap between electrodes is presented. This complete study proves that PMMA can be used as a HREBL negative resist to fabricate nanoelectrodes separated by a controlled and reproducible gap ranging from 5 nm to several tens of nanometers.

Nanospintronics: when spintronics meets single electron physics

As spintronics goes nano, new phenomena are predicted resulting from the interplaybetween spin dependent transport and single electron physics. The long term goal ofmanipulating spins one by one would open a promising path to quantum computing.Towards this end, there is an ever-growing effort to connect spin tanks (i.e. ferromagneticleads) to smaller and smaller objects in order to study spintronics in reduced dimensions.As the dimensions are reduced, spin dependent transport is predicted to interplay withquantum and/or single electron charging effects. We review experiments andtheories on the interplay between Coulomb blockade and spin properties (namelymagneto-Coulomb effects) in structures where a single nano-object is connected toferromagnetic leads. We then discuss briefly future directions in the emerging field ofnanospintronics towards quantum dots, carbon nanotubes and single molecule magnets.

Interconnection of specific nano-objects by electron beam lithography — A controllable method

We report a widely applicable and highly controlled approach, based on electron beam lithography (EBL), to interconnect single nano-objects, previously immobilized onto solid surfaces, and to investigate the transport properties at the level of single nanostructures. In particular, a three-step EBL-procedure was used for this purpose by patterning two planar contacts on the sides of an individual nano-object. To demonstrate this approach, we use two different kinds of active elements: a semiconductor nanocrystal (tetrapod) and a thin triangular gold nanoprism (NT).

Selective deposition of functionalized nano-objects by nanoxerography

Guided assembly of nanoscale objects on solid substrates via electrostatic attraction represents a simple and versatile process for the maskless fabrication of sub-μm structures and arrays of single nano-objects. To this end, patterns of local surface charges are written into an electret layer by applying voltage pulses to a conductive atomic force microscope tip. These charge patterns act as templates for the deposition of nanoscale objects in the subsequent development step. Here we describe the direct deposition of surface functionalized polystyrene beads and multiwalled carbon nanotubes from alcohol-based suspensions. Carboxyl groups lead to negative surface charges on suspended nano-objects resulting in Coulomb attraction to the positively charged templates. Assays using 100 nm diameter functionalized polystyrene spheres demonstrate that this nanoxerography method yields dense monolayers with excellent pattern definition and also allows for the precise arraying of single beads. Carboxylated carbon nanotubes deposit as dense layer of mainly single nanotubes on the predefined charge patterns.

Method of determining the eigenfrequencies of an ordered system of nanoobjects

A method is proposed to determine the eigenfrequencies of nanostructures (nanotubes and nanocrystals) by measuring the eigenfrequencies of a “large system” that comprises an array of vertically oriented similar nanotubes or nanocrystals equidistantly grown on a substrate. It is shown that the eigenfrequencies of a single nanoobject can be derived from the eigenfrequency spectra of the large (array-substrate) system and of the substrate. In other words, using experimental data for large systems, one can determine the eigenfrequencies of single nanoobjects, which are difficult to determine otherwise. By way of example, the eigenfrequencies of an array of zinc oxide micro-or nanocrystals on a sapphire substrate are calculated.

Optical response of a single noble metal nanoparticle

The characterization of a single metal nanoobject by comparing its theoretical and experimental far-field spectra measured by a spatial modulation spectroscopy (SMS) technique is discussed in the case of gold and silver nanoparticles. Quantitative determination of the polarization dependent absorption cross-section spectrum of a single nanoparticle is shown to permit its optical identification, i.e., determination of its shape, size and orientation on a surface.

ChemPhysChem: Ongoing Growth

Last year was marked by an outstanding growth of ChemPhysChem: the journal published about 2679 pages—a stunning 38 % more than in 2004 (see Figure 1). Similarly the number of published articles has gone up by 35 % (from 245 to 331). While the number of pages published per issue has more than doubled within the last three years, quality remained at our highest standards, as supported by ChemPhysChem's latest ISI Impact Factor of 3.596 and a rejection rate of 55 %. Page and submission development of ChemPhysChem over the years. Growth was also noticed in the submission rate. In 2005, ChemPhysChem received ca. 650 manuscripts, ca. 10 % more than in 2004. Scientists from more than 40 countries submitted to ChemPhysChem, with particularly large numbers from Europe (50%) and Asia (30%). Highlights of 2005 were three special issues: The May issue, co-organised by Professor P. Vöhringer and Dr. G. Haran, highlighted the inter-related fields of single-molecule spectroscopy and optical spectroscopy of biomolecular dynamics. This special issue evidently attracted enormous attention (see top five articles in Table 1). Title Corresponding author Page (issue) Single-Molecule Quantum-Dot Fluorescence Resonance Energy Transfer T. Ha 956 (5) Single-Molecule Fluorescence Spectroscopy of Protein Folding B. Schuler 1206 (7) Statistical Evaluation of Single Nano-Object Fluorescence M. Orrit 770 (5) Three-Color Single-Molecule Fluorescence Resonance Energy Transfer A. A. Deniz 74 (1) Proteins in Action Monitored by Time- Resolved FTIR Spectroscopy K. Gerwert 881 (5) With the Parrinello Festschrift (Figure 2), honouring our Editorial Advisory Board member Professor M. Parrinello on the occasion of his 60th birthday, ChemPhysChem published the fattiest issue ever. The Festschrift, organised by Dr. W. Andreoni, Professor D. Marx and Professor M. Sprik, included about 50 excellent contributions on modern computational chemistry and was extremely well received. Cover picture of the Parrinello Festschrift. The third special issue came out last November and was dedicated to Professor F. C. De Schryver, who has served as Co-chairman of ChemPhysChem's Editorial Advisory Board since its inception. This special issue, organised by Professor M. van der Auweraer on the occasion of Professor F. C. De Schryver's 65th birthday, covered such hot topics as photochemistry, single-molecule spectroscopy, femtochemistry and scanning probe microscopy (Figure 3). Cover picture of the special issue dedicated to Frans De Schryver. The five most downloaded ChemPhysChem articles, published in 2005 on Wiley InterScience, show further that single-molecule spectroscopy attracts a lot of attention these days and has obviously found a good home in ChemPhysChem (see Table 1). The Article on single-molecule quantum-dot fluorescence by T. Ha et al., featured on the cover of issue 5 (Figure 4), heads the list, followed by the Review articles on single-molecule spectroscopy of protein folding by B. Schuler and on single nano-object fluorescence by M. Orrit et al. (Figure 5). Close on their heels are the Articles by A. A. Deniz on single-molecule fluorescence resonance energy transfer and by K. Gerwert et al. on time-resolved FTIR spectroscopy of proteins. Cover picture of the special issue on single-molecule studies and biomolecular dynamics. Frontispiece of the Review article on single nano-object fluorescence by M. Orrit et al. Some of you may have used the new Citation Tracking feature launched on Wiley InterScience recently. This innovative feature allows to find content in a way not possible in the printworld. Unlike traditional reference linking which allows to see content quoted by the authors, citation tracking reveals links to content that cites the article as well. With related convenient tools we will continue to improve ChemPhysChem's web edition further. The ChemPhysChem team would like to thank our authors, reviewers, readers and the members of the Editorial Advisory Board for their continuing support over the last year. We wish you all a successful 2006 and look forward to exciting discoveries in chemistry, physics and biology some of which we hope to publish in ChemPhysChem. 1 Dr. Evelyn Wessel Deputy Editor PS: Apropos “ongoing growth”: We welcome our new sister journal ChemMedChem which is co-owned by the Società Chimica Italiana (SCI) and the Gesellschaft Deutscher Chemiker (GDCh) together with the other European chemical societies which form the Editorial Union of Chemical Societies (EUChemSoc). The editorial staff of ChemMedChem works closely together with the teams of ChemBioChem and ChemPhysChem. The new journal replaces Il Farmaco (published by the SCI until the end of 2005) and it will cover all aspects of drug discovery and development processes.

Interconnecting single nano-objects on surfaces for transport experiments

The authors report a highly controlled approach, based on electron-beam lithography, to interconnect individual nano-objects for transport experiments. The process is based on a three-step procedure, consisting of fabrication of four alignment markers, localization of the nano-object after its immobilization onto functionalized surfaces, and interconnection of the single nanostructure by patterning two nanoelectrodes on its sides. The approach is highly reproducible and widely applicable and allows an alignment accuracy of 15–20nm. Here they demonstrate the reliability of such technique by using a thin triangular gold nanoprism as the active element and show the I-V characteristics of the single nanostructure.

Fluorescent silver oligomeric clusters and colloidal particles

Highly luminescent single colloidal silver nanoparticles under blue excitation were detected. The intense luminescence observed from the single particles is attributed to different emissive centers. Picosecond lifetimes of those single nano-objects have been measured. Comparison with colloidal particles emission formed in aqueous solution, using polyacrylate polyanion as stabilizer is drawn. The optical properties of silver colloidal particles in solution are observed while their surface is changed by deposition of silver ion. A large visible emission under UV or surface plasmon region excitations is observed for Ag n @Ag +, while no fluorescence emission was found for Ag n or Ag + systems. The intense visible light emission changes should primarily come from chemical surface active sites. It was observed that the oligomeric clusters, composed of only few atoms Ag 4 + x x + and Ag 7 3 + , have a substantial emission in the same visible region. A model which ascribes the photoemission of the chemically modified silver colloidal particle to oligomeric clusters on the Ag particle surface is proposed. Those emissive water soluble silver clusters have potential for biological labelling, and light emitting sources in nanoscale optoelectronics applications.

Statistical Evaluation of Single Nano‐Object Fluorescence

Single nano-objects display strong fluctuations of their fluorescence signals. These random and irreproducible variations must be subject to statistical analysis to provide microscopic information. We review the main evaluation methods used so far by experimentalists in the field of single-molecule spectroscopy: time traces, correlation functions, distributions of "on" and "off" times, higher-order correlations. We compare their advantages and weaknesses from a theoretical point of view, illustrating our main conclusions with simple numerical simulations. We then review experiments on different types of single nano-objects, the phenomena which are observed and the statistical analyses applied to them.

SINGLE-MOLECULE OPTICS

We review recent developments in single-molecule spectroscopy and microscopy. New optical methods provide access to the absorption, emission, or excitation spectra of single nano-objects and can determine either the positions of these objects with subwavelength accuracy or the full three-dimensional orientation of their transition dipole moments. Recent work aims at using single molecules as nanoparts or nanoelements in a variety of molecular-scale devices, from triggered sources of single photons to single-molecular switches. A prominent new direction explores the various interactions between molecules within individual multichromophoric systems obtained by chemical synthesis. These systems are the models for natural self-assembled systems such as the light-harvesting proteins of bacteria and green plants, which are currently studied on a single-molecule basis. Another important class of multichromophoric systems are conjugated polymers. The combination of microscopy with time- and frequency-resolved spectroscopy is opening a wide field of new and exciting applications to individual nano-objects.

Room-temperature single nanoribbon lasers

Using a single nano-object measurement methodology that enables the correlation between size/morphology/structure and photoluminescence (PL) characteristics, we show that nanoribbons are an excellent model system to study single nano-objects. In particular, we measure the PL characteristics of optically pumped individual single-crystal zinc–sulfide nanoribbons. Small collection angle measurements show that nanoribbons form excellent optical cavities and gain medium with high (full width at half maximum<0.1 nm) lasing modes free of PL background even for a low pumping power density of 9 kW/cm2. Large collection angles add a broad PL component and obscure the correct high-quality lasing of the nanowires/nanoribbons.

Single Nanoobjects as Triggered Single Photons Sources

Single Nanoobjects as Triggered Single Photons Sources

Single Nanoobjects as Triggered Single Photons Sources

The generation of non-classical states of light [1] is an important scientific challenge with many potential applications. For example, squeezed states of light [2] have generated much interest in the past decade, because measurements can be performed with noise levels that are lower than those available with classical light. A particularly novel non-classical source of light is a triggered single-photon source: a source that has the property to emit with a high degree of certainty one (and only one) photon at a user-specified time. Such a deterministic source of single photons can be important for quantum information processing [3], quantum cryptography [4] and certain quantum computation problems [5]. A single quantum emitter such as a molecule or an atom is a good potential source because it can emit only one photon at a time. The emitted light is antibunched [6, 7] and consists of single photons separated by random time intervals, which depend on the excited state lifetime and on the pumping rate. Thus triggered emission of single photons can be obtained with controlled excitation of a single emitter. The first demonstration of a single photons source [8] using a single molecule in a solid is based on a rapid adiabatic passage method [9] to excite the molecule. In the effective spin picture, the Bloch vector follows the driving magnetic field adiabatically if the laser-molecule frequency detuning is swept slowly through the resonance. Since the passage inverts the field, the system goes from the ground to the excited state. From this fluorescent state the molecule can emit only a single photon. The molecular resonance is swept by Stark effect at fixed laser frequency. This is done easily by applying a radio frequency electric field to the molecule. The results showed photon states with statistics that differ radically from those of coherent states. But this method requires cryogenic temperatures (2K) since narrow zero phonon lines are needed for the adiabatic following. At room temperature a source of triggered single photons based on the concept of pulsed optical excitation of a single highly fluorescent molecule has been demonstrated [10]. A short pulse of green laser light pumps the molecule from the ground state to a vibronically excited level of the first electronic excited state. After fast (ps) relaxation to the lowest electronic excited state, the molecule subsequently emits a single photon. A frequency-doubled mode-locked Nd-YAG laser is used to pump highly stable single terrylene molecules contained in a p-terphenyl molecular crystal where they are protected from exposure to diffusing quenchers (such as oxygen). The result shows that the photons are emitted after the laser pulses at times separated by the laser repetition time within the lifetime of terrylene in p-terphenyl. Since the pump pulse width is very short compared to the fluorescence lifetime, the probability of two (or more) photon emission per pulse is always zero. At the maximum laser power, 90% of the laser pulses lead to a single photon emission. The parameters of this source (repetition rate and single photon generation probability) were limited only by the laser system used; nevertheless, this performance already surpasses that of previous work. The high performance combined with the vanishing two photon emission probability and the simplicity of this source suggests that it can be considered for a variety of quantum optical experiments and for secure quantum cryptography [11]. Semiconductor quantum dots (QD) are other potential systems for single photon generation [12]. QDs are often referred to as “artificial atoms” because many of their optical properties, such as ultranarrow transitions, arise from discrete, atomic-like energy levels [13, 14]. To reinforce this analogy a study of the photon antibunching in this system was needed. QDs differ from atoms and molecules since the creation of multiple excitons is possible in QDs. Consequently, if two electron-hole pairs can radiatively recombine, the simultaneous emission of two photons from a QD is then possible. Thus different regimes of photon statistics were expected because the probability of multiple exciton creation depends on excitation intensity. The fluorescence intensity correlation function of a single CdSe quantum dot has been investigated [15] using a coincidence setup (start-stop experiment). A strong photon antibunching has been observed over a large range of intensities (0.1-100 kW/cm2). The lack of coincidence at zero time delay indicates a highly efficient Auger ionization process [16], which suppresses multi-photon emission in these colloidal quantum dots. This result shows that room temperature single photon sources can be achieved with colloidal nanoparticles.

Polymerization of organized polymer assemblies

Supramolecular chemistry has provided the means by which to selectively assemble small and large molecules into intricate two- and three-dimensional nanostructures. The past 2 years have witnessed considerable increase in extending such self-assembled structures into stable, single macromolecular nano-objects, through intra-assembly covalent bond formation. The selective polymerization of specific domains within organized polymer assemblies allows for the preparation of crosslinked, nanostructured bulk or particulate materials, with a variety of possible morphologies and properties.