L.G. De Pietro

Evidence of nonplanar field emission via secondary electron detection in near field emission scanning electron microscopy

Nonplanar field emission from electrochemically etched tungsten field emitters has been observed using near field emission scanning electron microscopy. Close-proximity field emission in adequate ultrahigh vacuum conditions was implemented to attain Fowler–Nordheim plots using typical imaging parameters. The emission radii deduced via a detailed, spherical surface field emission theory, by [Edgcombe and de Jonge, J. Phys. D 40, 4123 (2007)], reveal that our sharpest tip asperities are less than a nanometer. This yields a spatial resolution on the order of one nanometer.

Fundamental Aspects of Near-Field Emission Scanning Electron Microscopy

Abstract In a previous publication (Kirk, 2010) the experimental technique of imaging near-field emission scanning electron microscopy (NFESEM) imaging was introduced. In NFESEM, a sharp tip in positioned at distances of a few 10nm from a metallic surface. Above a threshold voltage, electrons are field emitted from the tip. The field-emitted current is used, while scanning the tip across the surface at a well-defined, constant distance, to generate a topographic image of the surface with subnanometer vertical spatial resolution and a few-nanometer lateral spatial resolution. In this review, we discuss the fundamental physical processes that occur in NFESEM and provide some quantitative results. It is our goal to provide sufficient background information to allow NFESEM-based instruments to be developed in other laboratories.

Electron beam confinement and image contrast enhancement in near field emission scanning electron microscopy

In conventional scanning electron microscopy (SEM), the lateral resolution is limited by the electron beam diameter impinging on the specimen surface. Near field emission scanning electron microscopy (NFESEM) provides a simple means of overcoming this limit; however, the most suitable field emitter remains to be determined. NFESEM has been used in this work to investigate the W (110) surface with single-crystal tungsten tips of (310), (111), and (100)-orientations. The topographic images generated from both the electron intensity variations and the field emission current indicate higher resolution capabilities with decreasing tip work function than with polycrystalline tungsten tips. The confinement of the electron beam transcends the resolution limitations of the geometrical models, which are determined by the minimum beam width.

Improving the topografiner technology down to nanometer spatial resolution

In Scanning Tunnelling Microscopy (STM) the electrons are confined within the tunneling region, and this limitation has redirected scientists to alternative microscopy techniques, aimed at extracting the electrons away from the tunneling region. The topografiner - strictly speaking a precursor of STM, originally developed at the National Bureau of Standards - is an example. In this paper we report on the latest improvements of the topografiner technology that allow resolving topographic contrast with a lateral resolution down to 7 Å.

Scaling theory of electric-field-assisted tunnelling.

Recent experiments report the current (I) versus voltage (V) characteristics of a tunnel junction consisting of a metallic tip placed at a distance d from a planar electrode, d varying over six orders of magnitude, from few nanometres to few millimetres. In the ‘electric-field-assisted’ (or ‘field emission’) regime, as opposed to the direct tunnelling regime used in conventional scanning tunnelling microscopy, all I–V curves are found to collapse onto one single graph when d is suitably rescaled, suggesting that the current I=I(V,d) is in reality a generalized homogeneous function of one single variable, i.e. I=I(V⋅d−λ), where λ being some characteristic exponent and I(x) being a scaling function. In this paper, we provide a comprehensive explanation—based on analytical arguments, numerical simulations and further experimental results—for the scaling behaviour that we show to emerge for a variety of tip–plane geometries and thus seems to be a general feature of electric-field-assisted tunnelling.

Thirty per cent contrast in secondary-electron imaging by scanning field-emission microscopy

We perform scanning tunnelling microscopy (STM) in a regime where primary electrons are field-emitted from the tip and excite secondary electrons out of the target—the scanning field-emission microscopy regime (SFM). In the SFM mode, a secondary-electron contrast as high as 30% is observed when imaging a monoatomic step between a clean W(110)- and an Fe-covered W(110)-terrace. This is a figure of contrast comparable to STM. The apparent width of the monoatomic step attains the 1 nm mark, i.e. it is only marginally worse than the corresponding width observed in STM. The origin of the unexpected strong contrast in SFM is the material dependence of the secondary-electron yield and not the dependence of the transported current on the tip–target distance, typical of STM: accordingly, we expect that a technology combining STM and SFM will highlight complementary aspects of a surface while simultaneously making electrons, selected with nanometre spatial precision, available to a macroscopic environment for further processing.

Detecting the topographic, chemical, and magnetic contrast at surfaces with nm spatial resolution

Scanning tunnelling microscopy overshadowed other microscopy techniques owing to its unprecedented spatial resolution. However, the lack of secondary electrons in the experiment always motivated the quest for a complementary technique. The topografiner technology - a precursor of the STM - could not meet this task so far. Nevertheless, it still plays an important role in the arsenal of probing techniques. In this report, we present secondary-electron distributions of low-energy primary electrons directed at a cleaved GaAs(110) surface and preliminary measurements of single-energy surface imaging in a hybrid experiment.

The topografiner with energy analysis

The topografiner technology, developed originally at the National Bureau of Standards (now National Institute of Standards and Technology) with the aim of measuring the surface micro-topography, is less widespread than scanning tunneling microscopy but has a remarkable property: the electrons can escape the tip-surface junction. We have recently used topografiner imaging to map the surface of various metals and semiconductors with (almost) nanometer lateral spatial resolution. In this paper, we describe our attempt to endowing the NIST topografiner with an energy analysis of the electrons escaping the junction, with the aim of performing spectroscopy with nanometer spatial resolution.

Spin-polarised electrons in a one-magnet-only Mott spin junction

The current flowing through a Mott spin junction depends on the relative spin orientation of the two ferromagnetic layers comprising the “source” and “drain” sides of the junction. The resulting current asymmetry is detected as giant or tunnelling magnetoresistance depending on whether the two ferromagnets are separated by a metal or an insulator. Based on the fundamental principles of reciprocity for spin-dependent electron scattering, one can envisage a one-magnet-only spin junction in which the source is non-magnetic, and the spin information is encoded by the spin polarisation of the electrons that have crossed or are backscattered from the drain magnetic layer. The practical significance of using an unpolarised source is that the state of the magnetic layer can be modified without affecting the process of probing it. Whether this reciprocity is realised in the actual junctions is not yet known. Here, we demonstrate a nano-sized, one-magnet-only Mott spin junction by measuring the finite spin polarisation of the backscattered electrons. Based on this finding, we conclude that since the junction acts as a spin filter, the magnetic layer must experience a spin transfer that could become detectable in view of the high current densities achievable in this technology.

Detecting the topographic, chemical and magnetic contrast at surfaces with nanometer spatial resolution

The possibility of resolving magnetic-textures in real space at atomic scale may trigger novel fundamental and applicative perspectives. We report on energy-resolved surface imaging with a new technique called Near Field-Emission SEM, which confirms the feasibility of electron spectroscopy and magnetic-domain mapping with nanometer spatial resolution.