Electrical conductivity and photoluminescence properties of nanomaterials

11

Electrical conductivity and photoluminescence properties of nanomaterials

Electrical properties of nanoparticles

Introduction

            The electrical conductivity is a material- dependent property which for metallic conductors is independent of the applied voltage or the flowing electrical current. In contrast, for semiconductor or insulators the conductivity usually increases with increasing applied voltage.  When reducing the geometric dimensions of a wire to nanometer or molecular dimensions.  Ohms law is no longer valid in any case. Rather, the strictly linear relationship between current and voltage is replaced by a nonlinear, non-ohmic characteristic. In order to understand these phenomena, it is necessary first to consider the mechanism of electrical conductivity, the conventional macroscopic case.

                             V= I R = I (1 / G);    G = I / V            à1

Where V= Applied voltage; I= electric current

              R= Resistance        ; G= Electrical conductance

Note:  G depends on geometric parameters. (length & cross section). But σ (electrical conductivity) is material dependent property and it is independent of the applied voltage or the flowing electric current (for metal). For semiconductor σ usually increases with increasing the applied voltage.

 Conduction Mechanism of Bulk

Let us consider an electrical conductor such as metallic wire is concerned to an electrical circuit and electrons start to move, driven by the electrical field.  Within the wire there are a huge number of electrons moves slowly from one end of the wire to the other end.  In this way, the electrons experience scattering processes that lead to a change in the momentum by interactions with electrons, phonons, impurities or other imperfections of the lattice, which are responsible for the electrical losses. In metallic wires, electrical conductivity is characterized by mean free path of the electrons.  In an electrical field the electrons exhibit a type of "drift movement", and such a process of electrical conductivity is termed "diffusive conductance". Reducing the size of the conducting wire changes the mechanism of electrical conductivity.  Hence when the geometric dimensions reach the mean free path length of the electrons, the mechanism of conduction changes from a diffusive to a “ballistic”.

Conduction Mechanism of Nanoparticles

            In the ballistic conduction the scattering phenomena are no longer observed, classically zero resistivity is expected, but this is not observed because now quantum mechanical phenomena are occurring. In order to understand this ballistic conductivity Eq: 1 must be rewritten in a form which takes into account the transport of electricity by electrons. That is the electrical current I transports within a time interval ∆t the charge Q.

Ballistic conductivity of an electrical current in a small electrical conductor.  Ballistic conductivity is not characterised by scattering of the free electrons in the lattice, as the geometric dimensions of the conductor are smaller than the mean free path length of the electrons.

Finally we get                            G = (Ne²/h) × (λ/L)

            Here (L/λ) = n is the electron wave mode number.  Each electron wave mode can have two modes (spin up and spin down) leading to N = 2n; therefore, one finally obtains for the conductance of a short, thin wire with one mode            G = 2e² /h.  Assuming m active modes in a wire, the conductance is

G = 2me² /h.

            From above discussion, there are no longer any variables depending on the material or the geometry of the wire.  It is clear that the electrical conductance of a small, thin wire increases with the increment G₀ = 2e² /h = 7.72 ×10^-5S. Hence the conductance decreases with increasing voltage.

Electrical Parameters

Nanoparticles array shows environment–dependent electrical properties

(conductivity). These properties are modified by the chemical species present in its vicinity. The

Conductivity of nanoparticles is believed to occur due to:

1. Tunneling of electrons through the metal core.

2. Hopping of the electrons along the atoms constituting the chain of the legend molecule encapsulating the nanoparticle.

By changing the parameters of the nanoparticle such as its particle diameter, space between the particles and the number of layers, the conductivity of the system can be altered. The analyte can be made to interfere with any one of the processes and hence can help vary the conductivity. This could lead to a sensing of the analyte.

Application

            In recent years, several experimental groups have reported measurements of the current-voltage (I-V) characteristics of individual or small numbers of molecules.  Even three-terminal measurements showing evidence of transistor action has been reported using carbon nanotubes as well as self-assembled monolayer of conjugated polymers. These developments have attracted much attention from the semiconductor industry that are actively looking for ways to progress from gigabit to terabit integration by complementing or even replacing present day CMOS circuitry. There is great interest therefore from an applied point of view to model and understand the capabilities of molecular conductors.

            Let us consider for an example CNT. The unusual properties of carbon nanotubes make many applications ranging from battery electrodes, to electronic devices, to reinforcing fibres, which make stronger composites.

2.0       Optical Properties

The reduction of materials' dimension has pronounced effects on the optical properties. The size dependence can be generally classified into two groups. One is due to the increased energy level spacing as the system becomes more confined, and the other is related to surface plasmon resonance.

2.1       Surface plasmon resonance

Surface plasmon resonance is the coherent excitation of all the "free" electrons within the conduction band, leading to an in-phase oscillation. When the size of a metal nanocrystals is smaller than the wavelength of incident radiation, a surface plasmon resonance is generated and Fig. 6 shows schematically how a surface plasmon oscillation of a metallic particle is created in a simple manner. The electric field of an incoming light induces a polarization of the free electrons relative to the cationic lattice. The net charge difference occurs at the nanoparticles boundaries (the surface), which in turn acts as a restoring force. In this manner a dipolar oscillation of electrons is created with a certain frequency. The surface plasmon resonance is a dipolar excitation of the entire particle between the negatively charged free electrons and its positively charged lattice. The energy of the surface plasmon resonance depends on both the free electron density and the dielectric medium surrounding the nanoparticle. The width of the resonance varies with the characteristic time before electron scattering. For larger nanoparticle, the resonance sharpens as the scattering length increases. Noble metals have the resonance frequency in the visible light range.

2.2       Quantum size effects

Unique optical property of nanomaterials may also arise from another quantum size effect. When the size of a nanocrystal (i.e. a single crystal nanoparticle) is smaller than the de Broglie wavelength, electrons and holes are spatially confined and electric dipoles are formed, and discrete electronic energy level would be formed in all materials. Similar to a particle in a box, the energy separation between adjacent levels increases with decreasing dimensions. Figure 7 schematically illustrates such discrete electronic configurations in nanocrystals, nanowires and thin films; the electronic configurations of nanomaterials are significantly different from that of their bulk counterpart. These changes arise through systematic transformations in the density of electronic energy levels as a function of the size, and these changes result in strong variations in the optical and electrical properties with size. Nanocrystals lie in between the atomic and molecular limit of discrete density of electronic states and the extended crystalline limit of continuous band. In any material, there will be a size below which there is substantial variation of fundamental electrical and optical properties with size, when energy level spacing exceeds the temperature. For a given temperature, this occurs at a very large size (in nanometers) in semiconductors as compared with metals and insulators. In the case of metals, where the Fermi level lies in the center of a band and the relevant energy level spacing is very small, the electronic and optical properties more closely resemble those of continuum, even in relatively small sizes (tens or hundreds of atoms) . In semiconductors, the Fermi level lies between two bands, so that the edges of the bands are dominating the low-energy optical and electrical behavior. Optical excitations across the gap depend strongly on the size, even for crystallites as large as 10,000 atoms. For insulators, the band gap between two bands is already too big in the bulk form.

The quantum size effect is most pronounced for semiconductor nanoparticles, where the band gap increases with a decreasing size, resulting in the interband transition shifting to higher frequencies. In a semiconductor, the energy separation, i.e. the energy difference between the completely filled valence band and the empty conduction band is of the order of a few electrovolts and increases rapidly with a decreasing size. It is known that both the absorption edge and the luminescence peak position shift to a higher energy as the particle size reduces in the optical absorption and luminescence spectra of the InP nanocrystals. This size dependence of absorption peak has been widely used in determining the size of nanocrystals.

When the diameter of nanowires or nanorods reduces below the de Broglie wavelength, size confinement would also play an important role in determining the energy level just as for nanocrystals. For example, the absorption edge of Si nanowires has a significant blue shift with sharp, discrete features and silicon nanowires also have shown relatively strong "band-edge" photoluminescence.

2.3       Measurement method of optical properties of nanoparticles :

PL spectroscopy concerns monitoring the light emitted from atoms or molecules after they have absorbed photons. It is suitable for materials that exhibit photoluminescence, PL spectroscopy is suitable for the characterization of both organic and inorganic materials of virtually any size, and the samples can be in solid, liquid, or gaseous forms.

            Electromagnetic radiation in the UV and visible ranges is utilized in PL spectroscopy. The sample’s PL emission properties are characterized by four parameters: intensity, emission wavelength, bandwidth of the emission peak, and the emission stability. The PL properties of a material can change in different ambient environments, or in the presence of other molecules. Many nanotechnology-enabled sensors are based on monitoring such changes. Furthermore, as dimensions are reduced to the nanoscale, PL emission properties can change, in particular a size dependent shift in the emission wavelength can be observed. Additionally, because the released photon corresponds to the energy difference between the state, PL spectroscopy can be utilized to study material properties such as band gap, recombination mechanisms, and impurity levels.

In a typical PL spectroscopy setup for liquid samples is shown in Fig. 8, a solution containing the sample is placed in a quartz cuvette with a known path length. Double beam optics are generally employed. The first beam passes through an excitation filter or monochromator, then through the sample and onto a detector. This impinging light causes photoluminescence, which is emitted in all directions. A small portion of the emitted light arrives at the detector after passing through an optional emission filter or monochromator. A second reference beam is attenuated and compared with the beam from the sample. Solid samples can also be analyzed, with the incident beam impinging on the material (thin film, powder etc.). Generally an emission spectrum is recorded, where the sample is irradiated with a single wavelength and the intensity of the luminescence emission is recorded as a function of wavelength. The fluorescence of a sample can also be monitored as a function of time, after excitation by a flash of light. This technique is called time resolved fluorescence spectroscopy.

Nanomaterials with PL effects, in particular nanocrystals, can reveal many interesting and improved optical properties. These include brighter emission, narrower emission band, and broad UV absorption. For example, semiconductor nanocrystals produce narrower emission peaks than luminescent organic molecules, with bandwidth of around 30-40 nm. Having a smaller bandwidth, it is much easier to discriminate individual wavelengths emanating from multiple sources, such as in an array of nanocrystals. The PL emission intensities and wavelengths are dependent on particle size. Hence, PL spectroscopy directly enables particle size effects, in particular those in the nanoscale, to be observed and quantified.