- NANO EXPRESS
- Open Access

# Dimensional Effects on Densities of States and Interactions in Nanostructures

- Rainer Dick
^{1}Email author

**Received:**15 April 2010**Accepted:**7 June 2010**Published:**2 July 2010

## Abstract

We consider electrons in the presence of interfaces with different effective electron mass, and electromagnetic fields in the presence of a high-permittivity interface in bulk material. The equations of motion for these dimensionally hybrid systems yield analytic expressions for Green’s functions and electromagnetic potentials that interpolate between the two-dimensional logarithmic potential at short distance, and the three-dimensional *r*
^{−1} potential at large distance. This also yields results for electron densities of states which interpolate between the well-known two-dimensional and three-dimensional formulas. The transition length scales for interfaces of thickness *L* are found to be of order *Lm*/2*m*
_{*} for an interface in which electrons move with effective mass *m*
_{*}, and
for a dielectric thin film with permittivity
in a bulk of permittivity
. We can easily test the merits of the formalism by comparing the calculated electromagnetic potential with the infinite series solutions from image charges. This confirms that the dimensionally hybrid models are excellent approximations for distances *r* ≳ *L*/2.

## Keywords

- Density of states
- Coulomb and exchange interactions in nanostructures
- Dielectric thin films

## Introduction

*d*of spatial dimensions enters in a manner which is of direct relevance to technology is in the density of states. In the standard parabolic band approximation, this takes the form (with two helicity or spin states)

*d*-dimensional volume and per unit of energy. The corresponding dependence of the relation between the Fermi energy and the density

*n*of electrons on

*d*is

Variants of these equations (including summation over subbands) are often used for *d* = 2 or *d* = 1 to estimate carrier densities in quasi two-dimensional systems or nanowires, and the density of states plays a crucial role in all transport and optical properties of materials. Indeed, the obvious relevance for electrical conductivity properties in micro and nanotechnology implies that densities of states for *d* = 1, 2, or 3 are now commonly discussed in engineering textbooks, but there is another reason why I anticipate that variants of Eq. (1) will become ever more prominent in the technical literature. Densities also play a huge role in data storage, but with us still relying on binary logic switching between two stable states (spin up or down, charge or no charge, conductivity or no conductivity), data storage densities are limited by the physical densities of the systems which provide the dual states. We could (and likely will) drive information technology and integration much further if we can find ways to utilize more than just two states of a physical system to store and process information. Then, data storage densities should become proportional to energy integrals
of local densities of states. Equation (1) for *d* = 1 or *d* = 2 is certainly applicable for particles which have low energies compared to the confinement energy of a nanowire or a quantum well, but how can we effectively model particles which are weakly confined to a nanowire or quantum well, or which are otherwise affected by the presence of a low-dimensional substructure? In these cases, we can devise dimensionally hybrid models [1, 2] which yield e.g. densities of states which interpolate between *d* = 2 and *d* = 3 [3, 4]. This construction will be reviewed in Sect. 2. Based on the experience gained with dimensionally hybrid Hamiltonians for massive particles, we can also construct inter-dimensional Hamiltonians for photons which should be applicable to photons in the presence of high-permittivity thin films or interfaces. These models can also be solved in terms of infinite series expansions using image charges, and the merits of this approach can easily be tested. The case of high-permittivity thin films and testing the theory against image charge solutions will be discussed in Sect. 3.

## Dimensionally Hybrid Hamiltonians and Green’s Functions for Massive Particles in the Presence of Thin Films or Interfaces

We use the connection between Green’s functions and the density of states to generalize Eq. (1) for massive particles in the presence of a thin film or interface.

*H*with spectrum

*E*

_{ n }and eigenstates |

*n*, ν〉 is

Here, ν is a degeneracy index and the notation implies that continuous components in the indices (*n*, ν) are integrated. The first equation simply states the relation between the resolvent
of the Hamiltonian and the Green’s function *G*(*E*) which is normalized as lim_{
m→0,E→0}
*G*(*E*)|_{
d=3} = (4π*r*)^{−1}.

*G*(0) determines e.g. 2-particle correlation functions and electromagnetic interaction potentials, and the energy-dependent Green’s function

*G*(

*E*) determines e.g. scattering amplitudes for particles of energy

*E*. Application for resistivity calculations is therefore another technologically relevant application of Green’s functions. However, in the present section we are interested in this function because it also determines the local density of states in a system with Hamiltonian

*H*through the relation

Here, we explicitly included a factor 2 for the number of spin or helicity states, because the summation over degeneracy indices in (3,4) usually only involves orbital indices.

For our present investigation, the distinctive feature of the interface is that the particles move in it with an effective mass *m*
_{*}, while their mass in the surrounding bulk is *m*. We use coordinates
parallel to a plane interface, which is located at *z* = *z*
_{0}. Bold vector notation is used for quantities parallel to the interface, e.g.
and
.

*L*. If the wavenumber component orthogonal to the interface is small compared to the inverse width, |

*k*

_{⊥}

*L*| ≪ 1, i.e. if the de Broglie wavelength and the incidence angle satisfy λ ≫ 2π

*L*|cosϑ|, we can approximate the kinetic energy of the particles through a second quantized Hamiltonian

The interesting aspect of the Hamiltonians (5,6) is the linear superposition of two-dimensional and three-dimensional kinetic terms. The formalism presented here could and will certainly be extended to include also kinetic terms which are linear in derivatives, in particular in the interface term. This would be motivated either by a Rashba term arising from perpendicular fields penetrating the interface [5–11] of from the dispersion relation in Graphene [12–15]. However, for the present investigation we will use a parabolic band approximation in the bulk and in the interface.

*G*

_{0}(

*E*), which is also translation invariant in

*z*direction, to

*G*

_{0}(

*E*) in the axially symmetric mixed parametrization for later comparison. The equation

where the definition ℓ≡*m*/2μ = *Lm*/2*m*
_{*} was used. The ℓ-independent terms in (10) correspond to the free Green’s function *G*
_{0}(*E*) (8).

*d*= 2 or

*d*= 3. However, it reduces to either the two-dimensional or three-dimensional density of states in the appropriate limits, see Fig. 1. For large energies, i.e. if the states only probe length scales smaller than the transition length scale ℓ, we find the two-dimensional density of states properly rescaled by a dimensional factor to reflect that it is a density of states per three-dimensional volume,

This limiting behavior for interpolation between two and three dimensions is consistent with what is also observed for the zero-energy Green’s function in the interface, see equations (21–22) below.

It is intuitively understandable that the presence of a layer reduces the available density of states for given energy, or equivalently increases the Fermi energy for a given density of electrons. The presence of a layer generically implies boundary or matching conditions which reduce the number of available states at a given energy.

*L*, see also Fig. 2. In terms of effective particle mass, this means

## Electric Fields in the Presence of High-Permittivity Thin Films or Interfaces

*q*is an electric charge in a dielectric material of permittivity . The zero-energy Green’s function in

*d*spatial dimensions is given by

We cannot infer from the previous section that the zero energy limit of the inter-dimensional Green’s function calculated there also yields a dimensionally hybrid potential, because we were dealing with solutions of Schrödinger’s equation instead of the Gauss law. However, we can rederive the zero energy limit of that Green’s function from the Gauss law for electromagnetic fields in the presence of a high-permittivity interface.

Suppose we have charge carriers of charge *q* and mass *m* in the presence of an interface with permittivity
and permeability μ_{*}, We continue to denote vectors parallel to the interface in bold face notation,
,
, etc.

*L*|cosϑ|, we can approximate the system with an action

and the continuity condition *E*
_{
z
}(*z*
_{0} − 0) = *E*
_{
z
}(*z*
_{0} + 0).

We can therefore read off the solution from the results of the previous section with *E* = 0 and now
.

^{1},

*r*≪ ℓ and 1/

*r*behavior for large separation

*r*≫ ℓ of charges in high-permittivity thin films,

see also Fig. 2.

*z*

_{0}= 0 and recall that the solution for the potential of a charge

*q*at ,

*z*= 0 proceeds through the

*ansatz*

and symmetric continuation to *z* < −*L*/2.

*r*≳

*L*/2, where both the image charge solution and the analytic model show strong deviations from the bulk

*r*

^{−1}behavior. This is illustrated in Fig. 3 by plotting the reduced electrostatic potential for a charge

*q*, in the interface.

It is also instructive to plot the relative deviation between the dimensionally hybrid potential which follows from (20) and the potential (23) from image charges.

*r*≳

*L*/2, the dimensionally hybrid model is a very good approximation to the potential from image charges with accuracy better than 10

^{−2}if . For , the accuracy is still better than 4 × 10

^{−2}.

## Summary

An analysis of models for particles in the presence of a low effective mass interface, and for electromagnetic fields in the presence of a high-permittivity thin film, yields dimensionally hybrid densities of states (11) and electrostatic potentials (17,20) which interpolate between two-dimensional behavior and three-dimensional behavior. The analytic model for the electromagnetic fields is in very good agreement with the infinite series solution already for small distance scales *r* ≳ *L*/2, where the potential strongly deviates from the standard bulk *r*
^{−1} potential. At distance scales smaller than *L*/2, *r*
^{−1}, behavior seems to dominate again for the electrostatic potential, in agreement with expectations that for distances which are small compared to the lateral extension of a dielectric slab, bulk behavior should be restored. However, note that neither the inter-dimensional analytic model nor the solution from image charges is trustworthy for very small distances, because both models rely on a continuum approximation through the use of effective permittivities, but the continuum approximation should break down at sub-nanometer scales.

The most important finding is that interfaces and thin films of width *L* should exhibit transitions between two-dimensional and three-dimensional distance laws for physical quantities at length scales of order *Lm*/2*m*
_{*} or
, respectively. Interfaces with strong band curvature or high permittivity should provide good samples for experimental study of the transition between two-dimensional and three-dimensional behavior.

## Appendix: Solution of Eq. 9

*m*/2μ =

*Lm/2m**. Fourier transformation of Eq (26) with respect to

*k*

_{⊥}yields finally

It is easily verified that Fournier transformation yields again the result (26).

## Footnotes

^{1}Our notations for special functions follow the conventions of Abramowitz and Stegun [16].

## Declarations

### Acknowledgements

This research was supported by NSERC Canada.

**Open Access**

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## Authors’ Affiliations

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