Laboratory of Organic Matter Physics

Our researchers were involved in the research of electrical interfaces to two-dimensional semiconductors. The outcome of collaboration was a surprising finding, which was published in Nanomaterials. The two-dimensional semiconductor material under research was tungsten diselenide (WSe2), which has emerged as a promising ambipolar semiconductor material for field-effect transistors (FETs) due to its unique electronic properties, including a sizeable band gap, high carrier mobility, and remarkable on–off ratio. However, engineering the contacts to WSe2 remains an issue, and high contact barriers prevent the utilization of the full performance in electronic applications. Furthermore, it could be possible to tune the contacts to WSe2 for effective electron or hole injection and consequently pin the threshold voltage to either conduction or valence band. This would be the way to achieve complementary metal–oxide–semiconductor devices without doping of the channel material.This study investigates the behaviour of two-dimensional WSe2 field-effect transistors with multi-layer palladium diselenide (PdSe2) as a contact material. We demonstrate that PdSe2 contacts favour hole injection while preserving the ambipolar nature of the channel material. This consequently yields high-performance p-type WSe2 devices with PdSe2 van der Waals contacts. Further, we explore the tunability of the contact interface by selective laser alteration of the WSe2 under the contacts, enabling pinning of the threshold voltage to the valence band of WSe2, yielding pure p-type operation of the devices. See more in Nanomaterials.

Using atomic force microscopy, we probed growth of pentacene molecules on graphene that was fabricated by chemical vapor deposition (CVD) and transferred onto 300 nm-thick SiO₂ substrates. We observe that the folds on CVD graphene act as a potential barrier for the surface transport of pentacene molecules.  They are oriented either parallel or perpendicular to the folds, they are also predominantly oriented along the symmetry axes of graphene. Orientation of pentacene islands on graphene evaporated at room temperature has a wide distribution. On the contrary, most of the pentacene islands evaporated at 60°C are oriented at 30° with respect to the fold direction. In addition, we interpret the 3D growth of pentacene islands on graphene in terms of reduced polar components of surface energy on graphene investigated with contact angle measurements. The results were published in ACS Omega.

We measured record-high charge mobility in randomly oriented multilayered network of two-dimensional (2D) Ti3C2Tx (where Tx is the surface termination and corresponds to O, OH and F) was studied using time-of-flight photoconductivity (TOFP) method, which is highly sensitive to the distribution of charge carrier velocities. We prepared samples comprising Ti3C2Tx with thickness of 12 nm or 6-monolayers. MXene flakes of size up to 16 μm were randomly deposited on the surface by spin-coating from water solution. Using TOFP, we have measured electron mobility that reached values up to 279 cm2/Vs and increase with electric-field in a Poole-Frenkel manner. These values are approximately 50 times higher than previously reported field-effect mobility. Interestingly, our zero-electricfield extrapolate approaches electron mobility measured using terahertz absorption method, which represents intra-flake transport. Our data suggest that macroscopic charge transport is governed by two distinct mechanisms. The high mobility values are characteristic for the intra-flake charge transport via the manifold of delocalized states. On the other hand, the observed Poole-Frenkel dependence of charge carrier mobility on the electric field is typical for the disordered materials and suggest the existence of an important contribution of inter-flake hopping to the overall charge transport. Results were published in https://doi.org/10.1016/j.diamond.2023.109879

Direct alkaline ethanol fuel cells are showing promising improvements. In this brief communication, we have presented atomic force microscopy of fuel cell membranes, enhanced with graphene-oxide nanoparticles. More in https://doi.org/10.3390/ASEC2022-13819

We measured record high charge mobility in a mm-sized organic semiconductors. Semiconducting mesocrystalline bulk polymer specimens that exhibit nearintrinsic properties using channel-die pressing are demonstrated. A predominant edge-on orientation is obtained for poly(3-hexylthiophene-2,5-diyl) (P3HT) throughout 2 mm-thick/wide samples. This persistent mesocrystalline arrangement at macroscopic scales allows reliable evaluation of the electronic charge transport anisotropy along all three crystallographic axes, with high mobilities found along the π-stacking. Indeed, charge-carrier mobilities of up to 2.3 cm²/Vs are measured along the π-stack, which are some of the highest mobilities reported for polymers at low charge-carrier densities (drop-cast films display mobilities of maximum ≈10⁻³ cm²/Vs). The structural coherence also leads to an unusually well-defined photoluminescence line-shape characteristic of an H-aggregate (measured from the surface perpendicular to the materials flow), rather than the typical HJ-aggregate feature usually found for P3HT. The approach is widely applicable: to electrical conductors and materials used in n-type devices, such as poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (N2200) where the mesocrystalline structure leads to high electron transport along the polymer backbones (≈1.3 cm²/Vs). This versatility and the broad applicability of channel-die pressing signifies its promise as a straightforward, readily scalable method to fabricate bulk semiconducting polymer structures at macroscopic scales with properties typically accessible only by the tedious growth of single crystals. More in https://doi.org/10.1002/adma.202103002

Our group has been involved in an interesting study of the photoconductivity of solution-processed nanographene ad-layers. Sensitization of graphene with inorganic semiconducting nanostructures has been demonstrated as a powerful strategy to boost its optoelectronic performance. However, the limited tunability of optical properties and toxicity of metal cations in the inorganic sensitizers prohibits their widespread applications, and the in-depth understanding of the essential interfacial charge-transfer process within such hybrid systems remains elusive. Here, we design and develop high-quality nanographene (NG) dispersions with a large-scale production using high-shear mixing exfoliation. The physisorption of these NG molecules onto graphene gives rise to the formation of graphene−NG van der Waals heterostructures (VDWHs), characterized by strong interlayer coupling through π−π interactions. As a proof of concept, photodetectors fabricated on the basis of such VDWHs show ultrahigh responsivity up to 4.5 × 107 A/W and a specific detectivity reaching 4.6 × 1013 Jones, being competitive with the highest values obtained for graphene-based photodetectors. The outstanding device characteristics are attributed to the efficient transfer of photogenerated holes from NGs to graphene and the long-lived charge separation at graphene−NG interfaces (beyond 1 ns), as elucidated by ultrafast terahertz (THz) spectroscopy. These results demonstrate the great potential of such graphene−NG VDWHs as prototypical building blocks for high-performance, low-toxicity optoelectronics. The study is published in https://doi.org/10.1021/jacs.1c07615.

Our team has an open position for a PhD candidate. Highlights:

• State-of-the-art lab 
• Network of world-renowned researches 
• Full funding 
• Top career opportunity
• International team

Candidate will get strong experience in the area of fabrication and characterization of charge transport and electronic properties of two-dimensional materials such as graphene or graphene-related materials, and/or electronic charge transport characterization of thin films of advanced high-mobility organic semiconductors. The selected candidate will become a member of Laboratory of Organic Matter Physics (http://www-lfos.ung.si)

keywords:

2D heterostructures, organic semiconductors, sensors, FET, photodetectors, neuromorphic devices, time-of-flight photoconductivy, SNOM and AFM, photolithography, clean room.

The position is available immediately. 

Cordially invited to contact us here.