Single-layer MoS2 or molybdenite (which is made of molybdenum and sulphur) is a semiconductor with a direct bandgap (of 1.8 eV) that might be better than indirect bandgap silicon for making certain photonic devices. It could even rival "wonder material" graphene (which does not have a bandgap at all in its pristine state) in future electronic circuits. A direct bandgap is important for making devices like light-emitting diodes, solar cells and photodetectors, and any other photonic device that exploits electron-hole pair excitation, because devices made with direct rather than indirect gap semiconductors are more efficient.

The material also has good charge mobilities of greater than 100 cm2/Vs – and perhaps even up to 500 cm2/Vs. These values compare well to state-of-the-art silicon. And that is not all: being a van der Waals solid (made up of 2D sheets that are weakly bonded to each other), it is compatible with a variety of substrates – even transparent or plastic ones. Finally, single-layer molybdenite is only about 0.65 nm thick, which means that devices made from it can suppress so-called short-channel effects, so allowing for very thin transistors.

For such nanoelectronics applications, MoS2 films must be made in large quantities and be of high quality. A good way of measuring sample quality is to look at the amount of imperfections on its surface. Researchers mainly study point defects here, such as host atom vacancies and impurity atoms because these directly dope a semiconductor and shift its chemical potential.

There are, however, defects on the mesoscale (nanometre to micron lengths), such as grain boundaries that affect the local electronic properties of MoS2 films. Micron-sized irregular ad-layers, for example, grow over MoS2 monolayers and if the electrical properties of these layers are very different from those of the underlying layer, they could adversely affect the performance of a device made from the material.

AFM-based impedance microscope

It is no easy task to characterize such thin films in a non-destructive way, but a team of researchers led by Keji Lai has now used a new AFM-based impedance microscope (MIM) to investigate the local conductivities of MoS2 monolayers. MIM is the gigahertz frequency equivalent of near-field scanning optical microscopy (NSOM) but it measures the local electronic rather than optical properties of a material. Compared with conventional DC probes though, microwave frequencies are high enough so that no contact electrode is needed for imaging – which makes the technique non-invasive and non-destructive to the sample.

The measurement itself involves delivering an excitation signal at 1 GHz to a sharp nanosized metallic cantilever tip that scans the MoS2 film. The sample, with its specific permittivity and conductivity, modifies the effective impedance of the tip. This changes the reflected microwaves slightly, and these changes can then be detected by radio-frequency electronics to create an impedance map that contains information on the local electrical properties of the sample.

Like a microwave oven

“Interestingly, the local interaction between the microwaves and the material in our microscope is not that different from what happens in a normal microwave oven – the food heats up while the plate on which it is on stays cold,” explains Lai. “You do not want to put a metal fork inside the oven though because metals have very low impedance to microwaves.”

The samples studied in this work were prepared by sulphurizing MoO3. Scanning electron microscopy images of the films revealed small, isolated, round MoS2 islands and bigger flakes that were more triangular in shape. As individual triangles grow to around 30–50 µm in size, they start to merge, finally forming a nearly continuous film.

Two types of mesoscale defect

The researchers say that they clearly saw two types of mesoscale defect on their monolayer MoS2. The first are dendritic ad-layers and the second are zigzag-shaped grain boundaries. The dendritic ad-layers (which appear as a second layer of MoS2 in conventional optical measurements) conduct electricity 100 times better than the single layer. These defects should be clearly avoided when making devices from the film, says Lai. In contrast, the zigzag grain boundaries are more resistive than the crystalline grains – something that agrees with results from previous studies.

“There might be a positive side to these mesoscopic defects, however,” he tells nanotechweb.org. “One of the major problems for semiconductor devices is the Ohmic contact to metal electrodes. We might envisage using defects with better conductance to lay down contact pads that would serve as an intermediate ‘stepping stone’ to reduce the resistances to electrical charge injection and collection needed for a device to work.”

The team says that it is now trying to better understand how the mesoscale defects grow, so that it can better control their size, shape and where they appear in a sample. “We are also investigating what causes a meso-defect to have such a high conductivity and how to vary this conductivity thanks to post-growth annealing,” adds Lai.

The current work is detailed in Nano Letters.