Liquid-phase exfoliation (LPE) is a cheap, scalable and facile way to produce graphene nanosheets. However, what is gained in processability, is then lost in homogeneity of the resulting nanomaterial. This issue is particularly relevant in devices for which inter-nanosheet resistance plays a significant role in their intrinsic performance. For instance, piezoresistive nanosheet-based strain sensors have already been shown to be deeply influenced by network composition and morphology.

In this recent work by Caffrey et al., the influence of nanosheet thickness was investigated and correlated to the piezoresistance of printed graphene sensors. Firstly, the nanosheet suspension was prepared via LPE, different flake sizes were selected through liquid cascade centrifugation (LCC) and their thickness was then estimated via atomic force microscopy (AFM). The team produced different sensors via spray coating a network of graphene nanosheets with thicknesses between 3 and 20 nm (Fig. 1).

Fig 1. (A) Photograph of a printed sensor. (B) Raman spectra of sprayed graphene films inks of different size-selected nanosheets. (C)–(E) SEM images of networks composed of nanosheets with different size ranges. Reproduced from DOI: 10.1039/D4NH00224E with permission from the Royal Society of Chemistry.

The devices show a clear trend of increasing resistivity and gauge factor with increasing thickness. By using a simple model that correlates the network resistivity with nanosheet thickness, a new model that successfully correlates the gauge factor with thickness was obtained (Fig. 2).

Fig 2. Plot of gauge factor as a function of nanosheet thickness showing both experimental data and the model fitting. Reproduced from DOI: 10.1039/D4NH00224E with permission from the Royal Society of Chemistry.

The authors carefully analyzed the different contributions to the gauge factor and were able to differentiate between the effect of straining of the nanosheets themselves and of the inter-nanosheet junctions. From fitting this model, they observed that, interestingly, strain has a significant influence on nanosheet resistivity, which means that applied strain not only makes the flakes in the network slide on each other, but also induces a detectable deformation on the flakes themselves. Unexpectedly, the calculated nanosheets’ gauge factor was negative. This was attributed to a decrease in nanosheet strain with applied strain, possibly due to the relaxation of built-in strain during network formation or the release of point-to-basal plane contacts.

By applying a theoretical approach to experimental data, the authors were able to interpret and quantify the piezoresistive response in disordered graphene networks produced by LPE. Overall, this study presents a step forward in actively understanding the mechanisms behind the piezoresistive behavior of printed graphene sensors.

To find out more, please read:

Quantifying the effect of nanosheet dimensions on the piezoresistive response of printed graphene nanosheet networks
Eoin Caffrey, Jose M. Munuera, Tian Carey and Jonathan N. Coleman
Nanoscale Horiz., 2024, Advance Article

About the blogger

Sara Domenici is a PhD student at Politecnico di Torino (Turin, Italy) under the supervision of Prof. Teresa Gatti. She was born in Verona (Italy) in 1998. In 2020, she obtained her Bachelor’s degree in Chemistry at the University of Padova (Italy). In 2022, she completed the Double Degree Programme between the University of Padova and the Justus-Liebig University in Giessen (Germany), where she spent 12 months, and obtained a Master’s Degree in Chemistry. Her PhD project is focused on Janus two-dimensional materials for energy conversion, but she also works on hydrogel sensors and dye-sensitized solar cells (DSSCs).

The performance of nanomaterials is intricately linked to their size and combination form. Quantum dots (QDs), for instance, exhibit unique electronic and quantum properties due to their size effect. Moreover, the optical and electronic attributes of QD assemblies can be tailored by adjusting the size of individual QDs and their spatial arrangement within the assembly. Thus, it is imperative to explore suitable and universally applicable methods to prepare adjustable QDs and their assemblies.

In a recent study, Akter et al. devised mesoscopic QD assemblies using a novel bio-catalytic nanoparticle shaping (BNS) approach. Specifically, the authors employed L-lysine as a linker to assemble CdSe/CdS QDs, initially yielding ultra-large QD assemblies. Subsequently, these assemblies were catalytically cleaved by trypsin, resulting in mesoscopic QD assemblies (ms-QD) with a size of 84 nm (Fig. 1A). Relative to single QDs, the redshift and weakened emission observed in the photoluminescence spectrum of ms-QD suggest the presence of internal emission reabsorption processes within the assembly, which holds promise for leveraging energy or electron/hole transfer processes in the production of new optical materials.

Fig 1. (A) Synthetic pathway of ms-QD by the BNS method and corresponding TEM image, and (B) the synthetic scheme of organic molecule assemblies. Reproduced from DOI: 10.1039/D4NH00134F with permission from the Royal Society of Chemistry.

Additionally, the authors synthesized and investigated nanoassemblies of the organic molecule tetrakis(4-carboxyphenyl)porphyrin (Fig. 1B) using a similar method. They observed significant differences in reactive oxygen generation under light among the various assemblies, indicating the potential to modulate the molecular function of the particle’s core unit by altering the composition of connecting molecules.

In summary, this method demonstrates high versatility and can be employed for preparing assemblies of both QDs and organic molecules. Furthermore, it allows for the replacement of biological enzymes/substrates as needed to generate various nanoassemblies with unique physicochemical properties for further applications.

To find out more, please read:

Bio-catalytic nanoparticle shaping for preparing mesoscopic assemblies of semiconductor quantum dots and organic molecules
Rumana Akter, Nicholas Kirkwood, Samantha Zaman, Bang Lu, Tinci Wang, Satoru Takakusagi, Paul Mulvaney, Vasudevanpillai Biju and Yuta Takano
Nanoscale Horiz., 2024, Advance Article

About the blogger

Chao Wang is a postdoctoral fellow at Oregon State University and a member of the Nanoscale Horizons Community Board. His research focuses on multiple nanomedicines (especially metal-organic frameworks) and their theranostics for cancer, endometriosis and stem cell tracking.