Residual polymer stabiliser causes anisotropic conductivity in metal nanoparticle 2D and 3D printed electronics

Inkjet printing of metal nanoparticles (MNPs) allows for design flexibility, rapid processing and enables 3D printing of functional electronic devices through codeposition of multiple materials. However, the performance of printed devices, especially their conductivity, is lower than those made by traditional manufacturing methods and is previously not fully understood. Here, we revealed that anisotropic electrical conductivity of printed MNPs is caused by organic residuals from MNPs inks. We employed a combination of electrical resistivity tests, morphological analysis and novel 3D nanoscale chemical analysis of printed devices using silver nanoparticles (AgNPs) to show that the polymer stabiliser polyvinylpyrrolidone (PVP) tends to concentrate between vertically stacked AgNPs layers as well as at dielectric/conductive interfaces. The understanding of organic residues behaviour in printed nanoparticles reveal potential new strategies to improve nanomaterial ink formulations for functional printed electronics.

Despite significant interest in utilising MNP-based materials in 2D and 3D printed electronic devices, the lower and anisotropic intra-layer (planar) and inter-layer (vertical) conductivity of MNP layers, compared to bulk metals, limits device performance and hence uptake in industry and products. 4 The conductivity of MNPs layers are known to be dependent on sintering temperature and have been previously attributed to morphological changes and possible organic residues. 4,30,34 However, the detailed mechanism of low-temperature sintering of MNPs that leads to reduced conductivity remains to be fully understood.
Organic molecules are used in MNPs inks as stabilisers or capping agents to enable particle dispersion in low viscosity solvents, however, their residues are likely to hinder device performance, 34 even when present in very small amounts. Surface sensitive chemical analysis techniques of time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS), in combination with gas cluster ion beams (GCIBs), are a powerful toolset for depth profiling organic materials with high chemical specificity, sensitivity, and nanometre depth resolution, [35][36][37][38][39][40][41][42] chemical imaging of buried hybrid organic/inorganic interfaces 43,44 and characterisation of core-shell structures. [45][46][47][48][49][50][51] Here, we present a comprehensive study of the effect of the organic residues, such as stabilisers, and their localised distribution at the interface between printed layers on the functional performance of the printed devices before and after low-temperature sintering.
Our results provide new insights on routes to improve intra-layer AgNPs sintering and overcome functional anisotropy and hence improve uptake of this potential transformational technology. Our methodology is transferable to other nanomaterialbased inks and relevant for the development and exploitation of both 2D and 3D printed electronics.

Distribution of polymer stabiliser upon printing of AgNPs
To investigate the interface of AgNPs during the inkjet printing and pining process, we carried out high resolution chemical analysis of samples under a selection of printing conditions. As a result, we have found polyvinylpyrrolidone (PVP) on the surface of printed layers of AgNPs by means of the unambiguous identification of characteristic PVP signal 36,52 using a novel 3D orbiSIMS instrument 53 and XPS ( Fig. 1 and Supplementary   Fig. S1). PVP is a commonly used stabiliser, reducing agent, and shape-controlling agent for the synthesis of metal nanoparticles. 54,55 As a capping agent, it makes MNPs disperse well in aqueous and organic solvents due to its amphiphilic characteristics derived from the highly polar amide group within the pyrrolidone ring and hydrophobicity from the methylene backbone. PVP thus plays a vital role in dispersing and stabilising the MNPs in the solvents for stable ink formulation. 56 However, PVP does not completely decompose under typical multi-material jetting and sintering temperatures (up to 150°C) 5 and its residues are likely to remain. Understanding the state and distribution of PVP upon inkjet printing and post-deposition treatment of AgNPs is essential to develop solutions that minimise its impact on the performance of printed electronics. Inkjet printing of AgNPs starts with printing and then pinning the material by removing the solvent (Fig. 1a). All printing was conducted on a hot stage at 90 °C, which is sufficient for pinning the printed ink without triggering AgNPs sintering. This temperature is also low enough to prevent printhead damage and polymeric substrate degradation. The chemical mapping data from the top surface of a printed layer of AgNPs (Fig. 1b) and individually deposited droplets (Fig. 1c-d) show a 'coffee ring' effect where the solute (PVP) and suspending MNPs tend to flow towards its pinned contact line following the outward flow initiated by the combination of evaporation and contact-line pinning effect. 57,58 This is observed as a pattern with PVP-rich lines at the edge of each swath of MNPs ink across the entire surface (Fig. 1b). 57,59 The presence of PVP, even if at very low concentrations, can hinder the sintering of MNPs at the interface, leading to reduced conductivity in the inter-layer (vertical) direction. 4 In a 3D printing context, subsequently deposited AgNPs layers will therefore encounter a PVP-rich surface and these organic molecules may become trapped at the interface. We examined the three-dimensional distribution of PVP, via ToF-SIMS depth profiling, within a sample consisting of four successively printed layers of AgNPs ink. The results ( Fig. 2a-b) show that the top surface has the strongest PVP signal intensity, and that PVP signal was also detected through the bulk. Moreover, the PVP intensity oscillates with an average period of 282.8 nm and peaks at approximately 300, 600 and 900 nm depth (confirmed by fast Fourier transforms in Supplementary Fig. S5a) which, based on the single layer data (Supplementary Fig. S2), are the depths at which printed layer interfaces are expected to be located. This is consistent with our hypothesis that PVP concentrates at the interface between printed layers (Fig. 2d) and suggests that this happens during the layer-wise printing process. The high resolution XPS spectrum of N1s of the printed AgNPs (Fig. 2c) shows that the non-interacting form of PVP, presumably unbound PVP residues in the ink, (represented by C-N at 400 eV) is more abundant at the top surface, whilst the interacting form (represented by C-N-Ag at 398 eV) has a stronger signal in the bulk (~100 nm deep). 60 This, in conjunction with the ToF-SIMS depth profile (Fig. 2b), indicates that non or weakly-interacting PVP tends to migrate to the outer surface of the ink, forming a thin coating on the top surface upon solvent evaporation (Fig. 2d). However, non or weakly interacting PVP was also detected in between printed layers with relatively high intensity.
In order to obtain enhanced depth resolution of chemical information within the printed material, we carried out ToF-SIMS depth profiling using a lower beam energy and lower current for the etching ion beam (  Supplementary Fig. S3). The slightly higher length is attributed to nanoparticles packing and the oscillating pattern alternates between characteristic ions for silver (such as Ag + , 109 Ag + and Ag2 + ) and PVP (such as C4H6NO + and C6H10NO + ). We interpret that this is related to the core-shell structure of AgNPs/PVP and the irregularities in the signal arise from the average PVP distribution on the surface of several Ag nanoparticles within the ToF-SIMS analysis area.
In this nanoparticle-resolution depth profile, we tracked the signal of more specific secondary ions related to the cyclic amide of pyrrolidone (C6H10NO + ), linear hydrocarbon (C4H7 + ), the cyclic amide-silver (C4H6NOAg + ) and silver (Ag + ) and normalised their signals by their maximum intensity ( Fig. 3d-e). We observed that each ion signal peaks at different depths, revealing molecular-level information. These results give evidence of the interaction of PVP with the AgNPs surface ( Fig. 3a), in agreement with what has been previously predicted via molecular dynamics simulations [61][62][63][64] . Every data point in the ToF-SIMS depth profile represents only the first few nanometres of that depth level.; moreover, none of the secondary ion intensities decrease to zero within an oscillation, which is related to the fact that the capping polymer is present on the surface of all silver nanoparticles.

Influence of sintering temperature on the distribution of residual polymer stabiliser and electrical resistivity
Sintering is a commonly used post process to promote inter-particulate bridging as well as the removal of residual organics. To evaluate how this process affects chemical, morphological and electrical properties of printed samples, the AgNPs were deposited and pinned as both single-layer and 200 layers electrodes and post-processed in an oven at different temperatures (T) up to 500 º C. The resistivity of all samples was measured at room temperature (Fig. 4a,b,d), while their morphology was assessed by electron microscopy ( Fig. 4c and Supplementary Fig. S6). The planar resistivity (ρxy) of the sample with a single printed layer decreased dramatically after processing at 100 o C and the resistivity remains ~13 µΩ•cm for the samples sintered at the temperatures ranging from 100 o C to 200 o C. Micrographs of AgNPs show that the particles start bridging around these temperatures ( Fig. 4c.i,ii), hence forming conductive channels and leading to resistivity reduction. We noticed that ρxy of the single layer printed AgNP samples remains about one order of magnitude higher than expected for bulk silver (black dashed line in Fig. 4d). A further increase of processing temperature to above 300 º C results in an unexpected increase of ρxy for the single-layered samples due to inter-particle voids with diameters between 200 and 500 nm shown in detect residual PVP at the surface of printed AgNPs after processing (Fig. 4e-h). The results show that PVP is still present after sintering the printed samples at temperatures between 100 and 230 ºC, where resistivity is heavily anisotropic. Moreover, the proportion of non-interacting PVP (normalised ratio between XPS N1s peaks at 400 eV and 398 eV,

Residual PVP in a multi-material 3D printed device
We applied the same methodology to map residual PVP within a fully 3D printed encapsulated strain sensor that was produced via multi-material inkjet printing ( Fig. 5ab). The conductive channel consists of 5 printed layers of AgNPs processed using UV light and the dielectric material is tri(propylene glycol)diacrylate (TPGDA), represented by the secondary ion C3H3O + . Details about the device and printing method are reported in our previous work. 4,43 As both the dielectric and conductive layers are printed contemporaneously, temperatures for in-situ or post-processing must remain low to avoid damaging the organic material. Moreover, the glass transition temperature (Tg) of PVP is in the range 100~150 o C, depending on its molecular weight 55,65,66 , which may lead to migration of PVP within the surface and interface of AgNPs during the post-processing sintering. 67 Chemical depth profiling results (Fig. 5c,e) show that residual PVP is present at higher quantities at the dielectric/conductive materials interface, in agreement with the results shown in Fig. 2, where greater signal of PVP was detected at the top surface of AgNPs printed layers. The presence of organic residues at the interface between contemporaneously deposited materials is potentially deleterious to device performance where, specifically for this case, it may influence adhesion between the organic and inorganic materials. Furthermore, at the top TPGDA/AgNPs interface (Fig. 5c), silver is detected earlier in the depth profile than PVP. This is explained by TEM micrographs of a cross section of the device (Fig. 5d), which show interpenetration of AgNPs into the dielectric material. This is not observed at the bottom interface (Fig. 5e) because of the 12 order in which materials are deposited: AgNPs deposited on a solidified layer of organic material have less mobility than when a liquid organic material is deposited onto a layer of AgNPs. PVP is represented by C6H10NO + d TEM micrographs of a cross section.

Discussion
We showed that poor inter and intra-layer electrical conductivity in silver structures produced via 3D inkjet printing of AgNPs results from a combined chemistry and morphology interface evolution during the low-temperature sintering process. This has been achieved via the determination of the 3D spatial distribution of residual polymeric capping agents within multiple printed layers. The residual polymer leads to anisotropic conductivity reduction as we showed that such organic residues accumulate at the interface of vertically stacked printed layers. With a clearer understanding of the distribution of residual organic stabiliser within the printed layers, it is possible to consider optimisation strategies to overcome the functional anisotropy of inkjet-based 3D printed electronics. When printing multiple layers, it is not practical to remove the part from the printer after each layer, especially when multiple materials are deposited contemporaneously (Fig. 5). In that case, post-processing strategies should be converted into in-situ processing steps. When working with commercially available nanoparticulate ink formulations, the presence of a polymer stabiliser such as PVP is often inevitable, which therefore requires additional post processing methods to overcome functional anisotropy. We showed that higher sintering temperatures are not always beneficial, not only because of potential damage to the substrate or contemporaneously deposited materials, but also because AgNPs tend to coalesce, resulting in further planar conductivity reduction. Silver nanoparticles have been proven to be sintered with ultraviolet (UV) and infrared (IR) light, which makes it the most likely processing route for printing 3D macrostructures, 4,30 therefore, the understanding of how a stabiliser behaves upon printing and sintering of metal nanoparticles is essential to aid ink formulation routes with different stabilisers that can either be removed at lower temperatures or be breakable upon excitation using UV or IR sources. 68

Methods
AgNPs ink used in this work was purchased from Advanced Nano Products (SilverJet DGP-40LT-15C) and was used as-received. The nominal composition of the ink consists of 38.85 wt% of silver nanoparticles dispersed in triethylene glycol monomethyl ether (TGME) and other additives. Thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) of silver NP inks were performed simultaneously on a TA Q600 unit. surfaces. To reach the buried interfaces, the etching beam current had to be greater and acquisition time per level lower, which resulted respectively in limited depth resolution and lower signal to noise ratio to be able to detect PVP within layers of AgNPs in a similar manner to what is presented in Fig. 1 and Fig. 2).
ToF-SIMS data processing: All ToF-SIMS intensity maps were normalised by total ion counts to correct for topographic features. All intensity maps were produced using the simsMVA software. 69 For 3D renders, voxel intensities were normalised by total ion counts to account for topographic features and the z-scale was corrected based on the substrate signal. The final 3D representations were created by combining rendered isosurfaces ranging from 30% to 50% of the maximum intensity for each ion. Multivariate analysis was carried out using the mass as the variables and each spectrum in the depth profile sequence as the observations. For each depth profiling dataset, Surface Lab 7.1 (IONTOF GmbH) was used to perform an automated peak search on the total spectra restricted only to peaks with intensity higher than 100 counts and masses between 30 u and 300 u. Dead-time corrected peak areas were then exported for each sequential mass spectrum in the depth profile. Principal component analysis (PCA) was performed using the simsMVA software, 69 which yields components loadings with groups of peaks that shared the same depth profile (scores). Prior to PCA, data was Poisson scaled to account for heteroscedasticity. 70 Orbitrap secondary ion mass spectrometry (OrbiSIMS) was carried out using a 3D OrbiSIMS (Hybrid SIMS) instrument. 53 A 20 keV Ar3000 + GCIB of 20 µm diameter, delivering 3.5 nA (with duty cycle set to 70.4%) was used as the primary ion beam. Depth profiling data were acquired over areas of 300 × 300 μm 2 using random raster mode with crater size 382 × 382 μm 2 . The sample voltage was set at +57.5 V. Argon gas flooding was used to aid charge compensation and the pressure in the main chamber was FW carried out the resistivity tests supported by JI, LT and CJT. All authors provided critical feedback and wrote the manuscript.