Measuring the molecular origins of stiffness in organic semiconductors

Domain: earth_science
Published: 2026-01-13
Editorial outcome: major_revision

Abstract
Mechanical properties of organic molecular semiconductors are determined by a combination of chemical structure and solid-state packing. Measurements of nanoscale mechanical properties on molecular surfaces via atomic force microscopy (AFM) are particularly challenging as the very act of probing how stiff these surfaces are may perturb them, making it difficult to discern subtle differences in stiffness arising from changes in molecular composition. This work presents the first direct, experimental demonstration of the tunability in the nanomechanical properties for a family of molecular semiconductors resulting from systematic alkyl side chain substitution. While such tunability is intuitively expected, it is a subtle effect that is extremely difficult to detect and quantify reliably from nanoscale AFM measurements due to various spurious force contributions operating on such small length scales. Only after identifying and removing these spurious contributions is the underlying molecular-scale tailoring of mechanical properties observable. Confidence in the measured stiffness trend is reinforced through simulations based on density-functional theory (DFT) and molecular dynamics (MD).

Introduction
For several decades, academic and industrial research on organic semiconductors has focused on their optical properties, electronic properties, and thermal properties. Applications that stem from such research include solar and thermal energy harvesting, bioelectronics, as well as new characterisation techniques1–7. Organic semiconductors are known for their large-area flexibility and macroscopic bendability; unique selling points central to most applications. Although their macroscopic flexibility is taken for granted, their mechanical stiffness on the micro and nanoscale is only beginning to be investigated, thanks to the emergence of new experimental measurement techniques8.

Nanomechanical measurements are carried out using specific modes on atomic force microscopes that capture the complete time-extended force interaction between the cantilever tip and the surface under study. This force-distance curve quantifies the total tip-surface interaction in stages: (a) as the cantilever is driven towards the surface, (b) as it is brought into contact with the surface, (c) as it makes an elastic deformation to the top few nanometres of the surface, and (d) as it is retracted to large separation where the force interaction is zero9,10. Several properties can be acquired through the measured force-distance curves, including surface adhesion, surface deformation, indentation depth, nanoscale stiffness and, within reasonable model-based estimation, the Young’s modulus. Macroscopically, stiffness represents a resistance to deformation under an external force and is an ensemble property of molecules under equilibrium. Together with the wealth of existing knowledge on charge-carrier mobilities in organic semiconductors, precision measurements of stiffness enable a deeper understanding of how their electronic properties and mechanical properties correlate. Generalisations from time-tested theories that link the charge-carrier mobility with the elastic modulus, such as Bardeen and Shockley’s deformation potential theory published originally in 1950, may ultimately be deployed to understand the link between electronic properties and elastic properties in appropriately oriented high-mobility organic semiconductors11.

Nanomechanical measurements of molecular thin films are non-trivial and require significant attention to detail. Unlike typical optical, electronic, or thermal measurements of molecular semiconductors that do not involve contact-based surface deformation, nanomechanical measurements do. The very act of probing a molecular surface mechanically can perturb it, leading to inconsistent measurement estimates.

In this work, the link between nanoscale stiffness, studied through force-distance curve mapping on an AFM, and the chemical structure of the organic molecules that compose a lattice is explored. Specifically, we conduct a comparative study of nanoscale stiffness for a family of organic small molecule semiconductors based on the dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT) aromatic core. Artefacts that arise when performing AFM-based nanomechanical measurements are discussed, including the hidden nuances of nanoscale tip-sample interactions that are generally overlooked in routine measurements. Under careful measurement conditions, it becomes possible to discern minute differences in nanoscale mechanical properties of this family of materials. Mechanical properties predicted using molecular dynamics and density-functional theory show consonance with experiment. Finally, we conjecture as to how mechanical properties and electronic properties may be connected under specific conditions.

Results
Chemical structure and surfacetopography of DNTT and its derivatives
Figure 1 shows the organic molecules based on the DNTT structural unit together with their accompanying surface textures. The thin film samples were measured in the Quantitative Imaging (QI) mode of a JPK atomic force microscope (See Methods)12–15. The measured surface topographies were confirmed on a separate set of samples investigated on a Park Systems NX10 atomic force microscope. Besides unsubstituted DNTT, shown in Fig. 1a, we studied a phenyl di-substituted DNTT derivative (Ph-DNTT-Ph, Fig. 1b, sometimes called diPh-DNTT in the literature),16,17, DNTT with octyl sidechains (C8-DNTT-C8, Fig. 1c18, and two DNTT derivatives bearing chiral centres on their sidechains (Fig. 1d, e). In this work, we refer to the latter as RR-C8*-DNTT-C8* (sometimes called R-DNTT in the literature), and SS-C8*-DNTT-C8* (sometimes called S-DNTT in the literature)19. These molecular semiconductors were deposited using thermal evaporation (see Methods) at base substrate temperatures optimised for the highest charge-carrier mobilities in polycrystalline thin-film transistor devices (see caption of Fig. 1). It is clear from the surface topographies that each film’s growth is mediated by different modes, namely, Volmer-Weber or island formation as seen predominantly for Ph-DNTT-Ph, Frank-van der Merwe or layer-by-layer growth as seen predominantly for DNTT, Stranski-Krastanov or layer-plus-island growth seen in C8-DNTT-C8 (more islands and fewer terraces), SS-C8*DNTT-C8* (more terraces and fewer islands), and RR-C8*-DNTT-C8* (a hybrid between terraces and islands)20. It is known from ultrathin films of DNTT (a couple of nanometres thin) that continuous surface reorganisation takes place due to molecular migration up to several hours after the films have been deposited21,22. We have found that such surface reorganisation takes place on our films too, even though they are tens of nanometres in thickness. For this reason, we chose to probe the surface topographies of DNTT and all its derivatives shown in Fig. 1a few months after deposition. During this time, the samples were left sealed in vacuum packs, at room temperature, and were never exposed to ambient conditions from the time of their deposition. Right before measuring, each sample was removed from its vacuum pack and measured within 20 min of ambient exposure. Repeat measurements performed days later confirmed that there was no change in the surface topography. In what follows, we compare measurements of the nanoscale stiffness for these five organic molecular surfaces, optimised for charge-carrier mobilities, and demonstrate the impact of side-chain functionalisation on their nanomechanical properties.

Measurement of nanomechanics in DNTT and its derivatives
The nanomechanical properties of DNTT and its family of derivatives were measured using specific nanomechanical modes on an atomic force microscope (see Methods). This technique is fast becoming a way to probe the stiffness of materials on the nanoscale, with resolutions down to a couple of nanometres23,24. Figure 2a shows a schematic diagram of the cantilever together with its apex and demonstrates how the cantilever moves from one point to the next during measurement. The cantilever approaches the surface (along the line shown in blue), depresses the surface to a couple of nanometres, and retracts from the surface (along the line shown in red). The cantilever then moves to an adjacent pixel, and the same measurement is repeated. Upon contact with the surface, the apex of the rounded cantilever’s probe makes a viscoelastic deformation of the organic semiconductor surface, shown in Fig. 2b. The force is determined by the setpoint on the atomic force microscope, typically between 5 and 100 nN24–27.

The force-distance curve, necessary to extract the stiffness, is first mapped as a force-time curve shown in Fig. 2c. Considering the bending of the cantilever and the height of the z-scanner, one accurately determines the distance between the sample and the cantilever tip. Figure 2d plots the force as a function of traversed distance, rather than of time, and is called the force-distance curve. 1 is the region of pull-in when the cantilever tip is attracted towards the surface until it meets the surface. 2 is the point at which the attractive force and the repulsive force cancel, where we extract the topography of our film; this is called contact point imaging (CPI) in the Quantitative Imaging (QI) mode. Beyond this is the regime of repulsion, denoted as 3, where the cantilever is pressed against the surface. 4 is the point of maximum indentation (δ), determined by the chosen setpoint of the measurement, where the tip deforms the surface in the normal direction. As the cantilever tip is retracted from the surface, the force between the two follows the red line shown. When the cantilever tip is pulled away from the surface, it shows greater attraction to the surface due to surface adhesion, marked in Fig. 2d as 5. From here, one infers the adhesive properties of the surface.

Although several nanomechanical properties of the sample surface can be extracted from force-distance curves, we focus here on the stiffness of the surface (see Supplementary Information Sections 1, 2, 3, 4 and 5). This parameter is read from the force-distance curve as the slope of region 3, highlighted in Fig. 2d. The corresponding slopes are then mapped and compared across all the samples under study. Nanoscale stiffness represents a better point of comparison than Young’s modulus, since it is a directly measured quantity. The Young’s modulus is extracted based on the measured stiffness using contact models and the Poisson ratio. Figure 2e shows the configuration of measuring nanomechanics on the semiconductors in this study. DNTT is shown on the very left. In the case of DNTT, the monoclinic unit cell is compact but swells in volume, vertically, along the z-axis direction upon side chain substitution, i.e., in going from left to right in Fig. 2e to Ph-DNTT-Ph and C8-DNTT-C8. In RR-C8*-DNTT-C8* and SS-C8*-DNTT-C8*, the chiral side chains cause a slight expansion in the xy-plane. Figure 2f shows the herringbone packing of DNTT viewed from the top of the film, i.e., down the z-axis direction. This is the direction probed by the cantilever of the AFM and is related to the stiffness measured along the long axis of the conjugated molecular backbone. Within transistor devices, the charge-carrier mobility is measured in the xy-plane, shown in Fig. 2f, corresponding to the π-stacking direction within the herringbone structure. In this in-plane charge transport direction, ideas from deformation potential theory were previously used to connect transport with the modulus28.

Force-distance curves were measured at every pixel within a selected region of the samples to map stiffness spatially. We choose to do measurements on areas of 5 × 5 square microns, 1 × 1 square microns, and 300 × 300 square nanometres, with 512 × 512 pixels per scan for the larger areas, thus measuring the stiffness with a spatial resolution of ca. 10 nm. Using cantilevers with tip radii of about 20 nm, calibrated individually before acquiring each scanned image (see Supplementary Information Section 2), we obtain complete stiffness information of the materials from our scans. For smaller areas of 300 nm x 300 nm, we reduced the number of data points per scan. Higher resolution here makes the scanning step smaller than the radius of the AFM probe, and only increases the duration of the scan without offering any new insight.

Important subtleties in the nanomechanical measurements
The difficulty in performing nanomechanical characterisation of molecular surfaces lies in the fact that they are contact- and indentation-based, i.e., a surface must be ‘felt’ to probe its stiffness. Precise measurements of nanomechanical properties of molecular surfaces remain a fundamental challenge. Measurements rely on extremely fine contact and indentation, the signals of which are detected through changes in the position-sensitive photodiode (PSPD) response to AFM laser reflection. In other words, evaluating surface stiffness requires probing the surface in the intended direction, yet this process itself can perturb the mechanical properties of the target surface.

As described in Supplementary Information Section 5, we considered factors beyond typical plastic deformation or densification from indentation that could influence measurement outcomes. In particular, the potential lateral movement of the AFM tip and its impact on signal distortion were investigated.

Figure 3a–d present measurements on DNTT films, illustrating the importance of selecting an appropriate applied force. In Fig. 3a, forces exceeding 50 nN resulted in pronounced hysteresis between forward and retract curves, producing inconsistent and unreliable Young’s modulus values. Consequently, measurements were performed with forces below 50 nN.

Figure 3b shows indentation depth and adhesion measurements on DNTT using a clean AFM tip. Reliable stiffness values were obtained with indentation depths of about 2 to 4 nm, corresponding approximately to a single molecular layer, using the maximum indentation from Supplementary Fig. 6. Generally, increasing the force setpoint increased both indentation depth and adhesion, with adhesion ranging from 5 to 15 nN. Notably, at 50 and 70 nN, indentation depth plateaued and the expected correlation between adhesion and contact area πRd, where R is the tip radius and d is the indentation depth, was disrupted.

Figure 3c and the 3D model inset illustrate tip contamination after repeated scans. Although the contamination layer was only around 30 to 40 nm thick, the measured Young’s modulus was more than twice that of a clean surface. In Fig. 3d, the contamination resulted in up to a 30% reduction in indentation depth and a 30 to 100% increase in adhesion, indicating that contact occurred between organic aggregates rather than the original organic–inorganic interface.

Figure 3e shows adhesion fluctuations induced by tip contamination, suggesting that small molecules can detach during measurement, dynamically altering tip size and characteristics. These adhesion variations imply the presence of changing amounts of contaminant material between the tip and the sample, potentially enabling molecular-level lubrication under in-plane tip movement.

Figure 3f illustrates negative slopes in force-distance curves arising from tip contamination, which could be misinterpreted as “negative stiffness.” This may occur either due to actual tip motion causing a downward displacement despite decreasing load, or due to a lateral tip displacement during approach being misread as vertical bending during retraction. Both cases reflect abnormal tip behaviour and should be avoided for accurate nanomechanical assessment.

The plateau observed in Fig. 3a–d at high setpoints might appear attributable to densification under increased load. However, the negative slope during retraction in Fig. 3f cannot be explained solely by densification, as it is implausible for the tip to spontaneously move downward when the applied load decreases. Therefore, these distortions likely result from lateral tip movement or cantilever bending, which are interpreted as vertical movement of the cantilever.

To evaluate the lateral movement of the cantilever tip, additional FEM simulations were conducted (Fig. 3g–i). For a fully clamped cantilever, in-plane shear stress increased linearly with setpoint, and bending of the tip apex progressively increased from setpoints of 20 to 100 nN, becoming pronounced above 50 nN. In practice, weak adhesion (about 5 at 15 nN) is insufficient to maintain large bending, increasing the likelihood of lateral tip slip, which will alter the vertically measured signal on the PSPD.

Figure 3h, i further assesses the impact of lateral movement on these vertical signals. Using two simplified models, (i) modelling the cantilever and tip as rotating circles to simulate planar deformation, and (ii) representing the tip as a 2D arc, calculations showed that a lateral displacement of only 0.5 to 1 nm could be interpreted as vertical bending up to approximately 5 nm. For cantilevers with typical spring constants of 20 to 40 N/m, such bending would normally require 100 to 200 nN of applied load. Therefore, even minimal lateral movement can produce significant vertical signal artefacts.

In conclusion, contamination of the cantilever tip by detached DNTT molecules can severely distort measurement results. Contaminated tips reduce local contact pressure, potentially overestimating the Young’s modulus. Dynamic changes in tip morphology and size make post hoc SEM evaluation of these effects difficult. Notably, lateral tip movement may be misinterpreted as vertical displacement, producing artificial signals.

Overall, Fig. 3 emphasises that careful consideration of tip-surface interactions is essential in AFM-based nanomechanical measurements. Supplementary Information Section 5 covers these topics in more detail. Understanding these interactions is critical for accurately determining intrinsic stiffness trends of DNTT and derivatives, and for ensuring reproducibility and consistency across samples.

Comparison of nanomechanical properties for functionalised DNTT compounds
Figure 4 shows the measured surface topography and nanoscale stiffness of all five DNTT-based samples under investigation. Each of the rows in Fig. 4 show both the topography and stiffness at three different locations of the film, from the microscale to the nanoscale, as indicated by the scale bars. The stiffness of the family of films is investigated and compared in more detail in Fig. 5.

In Supplementary Information Section 3, we demonstrate that there is no strong correlation between the stiffness and the height topography in any of the DNTT derivatives. The fact that there is no clear correlation between stiffness and topographical roughness implies that the measurements are independent of each other and do not incorporate measurement artefacts from the way the tip scans across the sample during measurement. This is important to demonstrate, when assessing the reliability of nanoscale stiffness measurements. Often this can also be done by comparing line scans across topography and stiffness maps (see Supplementary Information Section 3) and ensuring that the features are not identical (which, for example, may be the case in samples with deep crevices where the AFM tip feels the texture around it more than it does underneath it).

As illustrated previously in Fig. 2e, alkylation on the DNTT molecule along the z-direction causes expansion in that direction. This leads to a reduction in molecular density, measured in molecules per cubic nanometre, as shown in Fig. 5a. The molecular density shown was calculated from the parameters of the unit cells of each material, tabulated in Supplementary Information Section 4. Along the vertical z-direction in which the stiffness is measured, a unit cell of DNTT contains only stiff cores, while the unit cell of C8-DNTT-C8 contains stiff cores with flexible alkyl side chains. The unit cell of Ph-DNTT-Ph sits intermediate between the two, owing to its shorter phenyl groups. In other words, when longer side chains are added onto the stiff DNTT core molecule, the unit cell expands in the vertical z-direction, the flexible side chains begin to occupy more volume within the unit cell, and the molecular density reduces. It must be reiterated that the unit cells are monoclinic with vertical c-axes being nearly perpendicular to the substrate’s xy-plane, and so very close to the substrate’s z-axis. The substrate’s xy-plane is parallel to the ab-plane of the monoclinic unit cell. RR-C8*-DNTT-C8* and SS-C8*-DNTT-C8*, having two extra methyl groups in C8-sidechains, expand slightly into the ab-plane (see Supplementary Information Section 4), causing an ever so slight volumetric expansion beyond that of C8-DNTT-C8.

The unit cell expansion along the z-direction upon alkylation was also observed through out-of-plane X-ray diffraction measurements summarised in Fig. 5b. Although synchrotron-based measurements of structure are customary in the characterisation of organic systems for their high brilliance and angular beam spread, lab-based X-ray diffraction measurements offer insights with rapid turnaround and proved useful when studying the structural properties of DNTT19,29. From Fig. 5b, the peak originating from out-of-plane stacking is seen to shift to the left when going from DNTT to Ph-DNTT-Ph, and even further for C8-DNTT-C8. This is confirmation that the unit cell expands in the vertical direction upon side chain substitution (seeing as the molecules stand edge-on the substrate with side chains directed nearly along the z-axis perpendicular to the substrate)30. The unit cells of RR-C8*-DNTT-C8* and SS-C8*-DNTT-C8*, based on chiral C8-sidechains, slightly contract in the vertical direction with respect to C8-DNTT-C8, as confirmed in Fig. 5b.

Figure 5c shows violin plots of stiffness measured in nN/nm, appropriately highlighting the scale at which measurements are being done. There are three violin plots per molecular structure, reflecting three different resolutions at which the measurements were done (see Fig. 4) namely, 5 μm × 5 μm, 1 μm × 1 μm, and 300 nm × 300 nm. In Fig. 5c, there is agreement between the average values of stiffness measured at each length scale confirming reproducibility across length scales. The contribution from domain boundaries does not skew the average values. From Fig. 5c, the stiffness of DNTT is larger than the stiffness of Ph-DNTT-Ph, which is in turn larger than the stiffness of C8-DNTT-C8. RR-C8*-DNTT-C8* and SS-C8*-DNTT-C8*, namely the chiral versions of C8-DNTT-C8, are shown to be the least stiff and are comparable with each other.

The underlying reason why substituted DNTT derivatives are softer when probed along the vertical direction is because the incorporated flexible side chains take up part of the volume in the unit cell. The π-conjugated core is hard, owing to its double bonds, while the alkyl side chains are soft. The combination of the two within a C8-DNTT-C8 unit cell renders it softer than DNTT. The stiffness trend in Fig. 5c is consonant with the changes in crystal density seen in Fig. 5a.

Figure 5d shows the measured surface adhesion forces on each film. The surface adhesion forces are understood to represent how sticky the samples are on the nanoscale. For measurements on stable films of the DNTT-family, the adhesion forces were comparable as shown in Fig. 5d. Had this not been the case, the stiffness values would not have been consistent or reliable because different surface adhesion can skew the portion of the force curves from which stiffness is extracted. Consistent adhesion values across all samples ensured reliability in the stiffness trend reported.

Numerical calculations of mechanical properties in DNTT and C8-DNTT-C8
To understand the trend in our measurements of stiffness shown in Fig. 5c, we undertook numerical calculations of the Young’s modulus. We identify the molecular films of DNTT and C8-DNTT-C8 as representing the most critical cases and calculate the modulus along all three orthogonal axes within their films. With these calculations, we can compare our out-of-plane AFM-based nanomechanical measurements with the calculated modulus along the z-axis.

Density-functional theory (DFT) calculations of the Young’s modulus were carried out for DNTT and C8-DNTT-C8 based on the theory of elasticity31. These calculations used the FHI-aims software32–34, and employed the B86bPBE functional35,36, and the XDM correction for describing van der Waals interactions37,38. First, the experimental crystal structures were fully optimised, relaxing both the atom positions and unit-cell parameters. A series of strained unit cells39 were then generated from the optimised cells, and the corresponding stresses were computed (see Supplementary Information Section 6)40.

To complement DFT calculations by including thermal and disorder effects, we also performed classical all-atom Molecular Dynamics (MD) simulations of the mechanical properties on DNTT and C8-DNTT-C8 using a modified Generalised Amber Force Field (GAFF)41,42 (see Supplementary Information Section 7 for details on the parametrization procedure). Starting from DNTT43 and C8-DNTT-C844 unit cells, two supercells containing 62 molecules each were created. We computed the stress tensor σ for different uniaxial strains (ε, up to ±1.0% with a 0.5% interval) and obtained the stiffness tensor C from Hooke’s law. In both DFT calculations and MD simulations, from the stiffness tensor, we deduced the Young’s modulus and the Poisson’s ratio along any desired direction, by least squares fit of the σ=C·ε relationship. Further computational details can be found in the Supplementary Information Sections 6 (DFT) and 7 (MD).

From the MD simulations, we observed for both DNTT and C8-DNTT-C8 that during contraction or expansion of the x-axis, the intermolecular distances P, T, and T , defined in Fig. 6a and plotted in Fig. 6d–f, increase (decrease) upon expansion (compression) of the unit cell. On the contrary, the distances P and P,(see Fig. 6a, b), being perpendicular to the strain direction, are not affected by the deformation along x and are therefore not shown (see Supplementary Fig. 41). The same analysis was performed along y and z with similar conclusions, namely that the intermolecular distances perpendicular to the strain direction are not affected by it, while the evolution of the other characteristic distances follow the increase (decrease) of the strain magnitude, as displayed in Fig. 6c, g, h, i.

The Young’s moduli computed at the DFT and MD levels are given in Table 1 and the values obtained with both methodologies are in broad agreement with each other. For the MD simulations, the standard deviations we computed for the E , E and E components are lower than 1%. The small differences in the xy-plane for C8-DNTT-C8 stand out; however, variations of this magnitude can be registered also by changing the functional or the force field parameters. Anisotropy of the elasticity is more robust to potential energy function changes45. It is worth noting that MD simulations are carried out at 300 K while DFT calculations are performed at 0 K: the inclusion of the kinetic energy in the MD simulations might permit exploring configurations, particularly for the flexible side chains in the xy-plane, that make the materials softer, and then explain the lower values obtained with MD for C8-DNTT-C8.

In comparing the mechanical properties of the two materials, clearly the DNTT crystal exhibits a larger resistance to deformations compared to C8-DNTT-C8 along the z-direction, consistent with the larger stiffness measured for the former system (see Fig. 5c). The larger E for DNTT is in line with what was observed in previous calculations carried out with bare and alkylated benzothieno[3,2-b]-benzothiophene (BTBT)46 and is attributed to the flexibility of C8 alkyl chains. The larger values of E with respect to E and E in both systems are explained by considering the parallel orientation of the molecular long axis with respect to the cell z-axis.

Table 1 also shows that, unlike E, the moduli along the x- and y-directions increase when going from DNTT to C8-DNTT-C8, although to a much greater extent with DFT than with MD. In both systems the elastic responses in the xy-plane show nearly identical values, i.e., E ≈ E, not surprisingly in view of the similar nearest-neighbour molecular interactions within the ab-(xy) plane (see Fig. 2f). The similarity of the in-plane Young’s moduli is substantiated by observing that the slopes of characteristic distances as a function of the x- and y-strain, obtained with MD simulations, are similar overall (see Fig. 6). Instead, the significant difference in E is accompanied by a larger variation of P versus ε for C8-DNTT-C8 as compared to DNTT (see Fig. 6c).

Overall, our MD and DFT results predict a large anisotropy in the Young’s modulus and report an E agreeing with experimentally measured trends in the stiffness.

Methods
Materials
DNTT was purchased from Sigma-Aldrich and used as received. C8-DNTT-C8 and Ph-DNTT-Ph were supplied by Luminescence Technology Corp. (Lumtec) and Nippon Kayaku. RR-C8*-DNTT-C8* and SS-C8*-DNTT-C8* were synthesised according to previously described procedures19.

Thin film deposition
The organic semiconductor thin films were fabricated on highly doped silicon wafers with 300 nm of SiO (Christian-Albrecht University of Kiel, Institute for Electrical Engineering and Information Technology). Organic semiconductors were evaporated in vacuum (UNIVEX 300, Leybold GmbH, pressure of ~10−6 mbar, deposition rate of 0.3 Å s−1 for DNTT and of 0.1 Å s−1 for Ph-DNTT-Ph, C8-DNTT-C8, RR-C8*-DNTT-C8* and SS-C8*-DNTT-C8*, nominal thickness of ca. 50 nm, monitored by a quartz crystal microbalance) onto the substrates which were held at a temperature of 80 °C for DNTT, 90 °C for Ph-DNTT-Ph, 100 °C for C8-DNTT-C8, and at 40 °C for RR-C8*-DNTT-C8* and SS-C8*-DNTT-C8*. Immediately following the deposition of OSCs onto the substrates, the samples were promptly sealed in vacuum packs at room temperature. They remained shielded from ambient conditions and light throughout the duration between deposition and nanomechanical measurements.

Nano-mechanical measurements
All AFM measurements were performed with the NanoWizard® 3 AFM (Bruker, USA) in the ambient at 23 °C. Nano-mechanical measurements using Quantitative Imaging (QI) were conducted on all samples with Tap300DLC cantilevers (Budget Sensors, Bulgaria) in areas of 5 by 5, 1 by 1, and 0.3 by 0.3 µm². Additional measurements were done on a Park Systems NX10 AFM. To avoid contamination, a new cantilever was used for each sample, and the normal bending spring constant and sensitivity was determined for each cantilever before the measurements. The normal bending spring constant using the Thermal Tune method was approximately 24 N/m. QI map analysis was performed using the JPK Data Processing software (v6.1.120), and Gwyddion (v2.64).

XRD measurements
Grazing Incidence X-ray Diffraction (GIXRD) analyses, both in-plane and out-of-plane, were conducted using a multi-purpose X-ray diffractometer (SmartLab, Rigaku, Japan) using Cu Kα (λ = 1.5406 Å) radiation generated by an X-ray generator at 45 kV and 200 mA. We show only the out-of-plane data in the work since it is relevant to our discussion.

Field effect transistor fabrication and electrical measurements
Field effect transistors with DNTT, C8-DNTT-C8 and chiral DNTTs were fabricated on highly doped silicon wafers coated with 30 nm of Al O (Christian-Albrecht University of Kiel, Institute for Electrical Engineering and Information Technology). The Al O substrates underwent oxygen plasma treatment (Diener Electronic; oxygen flow rate 20 sccm, pressure 0.50 mbar, plasma power 100 W, duration 2 min) and then immersed overnight in 1.5 mM solution of n-tetradecylphosphonic acid (TDPA, Sigma-Aldrich) in 2-propanol (Acros Organics) to obtain a 1.5 nm-thick self-assembled monolayer (SAM), resulting in an Al O /SAM dielectric possessing a capacitance (C) of 185.5 nF cm−2. Subsequently, the substrates were rinsed first in 2-propanol then in deionized water and finally in 2-propanol again and dried on a hot plate at 100 °C for 10 min. To fabricate bottom-gate bottom-contact (BGBC) devices, gold contacts were deposited on the gate-dielectric substrates at room temperature via vacuum deposition (UNIVEX 300, Leybold GmbH; pressure of 10-5 mbar, deposition rate of 0.5 Å s−1, nominal thickness of 50 nm, monitored using a quartz crystal microbalance). Afterward, the substrates were immersed in a 10 mM pentafluoro benzenethiol (PFBT, Alfa Aesar) solution in 2-propanol for 30 minutes to form a SAM on the gold bottom contacts. The substrates were then rinsed with 2-propanol and dried. OSC was deposited onto the substrates by vacuum evaporation (pressure of 10−6 mbar, deposition rate of 0.3 Å s−1 for DNTT and 0.1 Å s−1 for C8-DNTT-C8, RR-C8*-DNTT-C8* and SS-C8*-DNTT-C8* nominal thickness of 25 nm, monitored by a quartz crystal microbalance and confirmed by AFM measurements) using a shadow mask, with the substrates maintained at specific temperatures (70 °C for DNTT, 100 °C for C8-DNTT-C8 and 40 °C for RR-C8*-DNTT-C8* and SS-C8*-DNTT-C8*). This process yielded the final devices with a channel width (W) of 480 μm and a channel length (L) of 215 μm. Electrical measurements were performed at room temperature in ambient air using an Agilent 4155C Semiconductor Parameter Analyzer. The transfer characteristics were measured in the linear regime at a drain-source voltage (V ) of −0.1 V. Field-effect mobility was calculated using the conventional gradual channel approximation model with equations applicable to the linear regime:

μ = (L / W C V ) * (∂I / ∂V )

where I is the drain current and V the gate voltage. The IV characteristics and mobilities are summarised in the Supplementary Information Section 8.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The article by Kwang and coworkers addresses an important but underexplored question, which is the relationship between the molecular structure of an organic semiconductor film and its nanomechanical properties, particularly its stiffness. The work is carefully done and explained and the manuscript is clearly written. However, with the current version of the article I find it hard to recognise what exactly its novel contribution is.\n\nThe title suggests that the article is addressing the relationship between molecular structure and stuffness. To some extent the paper addresses this question and shows that in crystalline molecular thin films that are oriented with the long molecular axis normal to the substrate, that the stiffness along this axis depends on the amount of non-conjugated material along that axis, with longer side chains leading to lower stiffness for a given conjugated unit. This makes sense given that the fused ring pi-conjugated part of the molecule is itself stiffer than the more flexible end groups and given that the selected materials are fairly crystalline (so the molecular orientation is more or less known). This result is clear, though not surprising.\n\nThe abstract suggests that the article concerns the relationship between stiffness and electronic (charge transport) properties. Here the relationship is not so clear. Stiffness is measured in the z direction, FET mobility is measured in the xy plane. The authors explain the higher mobility of C8-DNTT than DNTT in terms of a higher calculated stiffness in that plane, but this has no relationship to their careful measurements of stiffness in the orthogonal direction and moreover the difference in calculated stiffness in the xy plane between the two materials is not larger than the error on the theoretical method that the authors tell us to expect. As well, they show very clearly that the calculated stiffness in the z direction is significantly larger than any orthogonal direction and yet, charge carrier transport is by far the slowest in that (z) direction, leaving the conclusion that high stiffness can lead to high mobility rather undermined. Previous work rationalising the higher mobility of C8-DNTT than DNTT appealed to electron-phonon coupling yet the theoretical methods that the authors select here to explain their results do not, as far as I can see, account for electron-phonon coupling. The authors argue that electron-phonon coupling is not the only parameter that matter sand here I am sure they are right, but I don’t find it very clear from the paper in its current form what physical explanation they are offering us instead.\n\nI like what the authors are trying to do but I think the authors need to clarify their story before I could recommend it for publication. One approach might be to improve the margins of error on the theoretical results by validation of the method on other material systems. It would be helpful if they could include the impact of vibrational modes in that analysis, or discuss them explicitly if they are already included in the work done. Another approach might be to find a way to probe transport and stiffness in the same directions. Or to focus the work primarily on structure-nanomechanical property relations, where the paper and the extensive Supporting information are very strong.\n\nA final general concern is that all the analysis is done assuming that the molecular films are crystalline are oriented in a particular sense, however we don’t know how crystalline they are. One might expect that any amorphous material and / or grain boundaries would affect both nanomechanical measurements and mobility. Can the authors say how crystalline the films are and can they quantify the relative degree of crystallinity of the different DNTT derivatives. Can they rule out the possibility that differences between materials could be caused by different degrees of crystallinity or orientation?\n\nApart from these general concerns I have some minor points:\nThe term ‘pi-conjugation’ is used several times in a way that suggests the authors mean pi stacking. I would understand that the direction of pi-conjugation is the long axis of the molecule, along which conjugation occurs, not an orthogonal direction.\nOn p.10 the authors stress that there is no correlation between nanoscale stiffness and topography yet the figure 3 seems to be showing that there is some correlation in the spatial features. If the apparent similarities between these images are due to some larger scale features like cracks then it would be helpful to remark on that when discussing the comparison.\np.11, please say briefly how density is determined. I am guessing this refer to a density worked out from the solved crystal structure and not a measurement that is relevant to the thin films.\nP12, is it not trivial that molecular density reduces as side chains get longer?\nThe x-ray diffraction data on p.14 are presented as showing the crystallinity of the films – does that mean degree of crystallinity and if so how? They are also claimed to rule out polymorphism but that may be too strong, depending on the sensitivity of the measurement.\nI would dispute that the DFT and MD are really in broad agreement with each other. Do you have good reasons to expect the methods to agree? For example did you use the same functionals that were selected for DFT to tune the force field used in MD?\nThe sentence on page 4 that tells the reader that they should have confidence in the work because the authors have plenty of experience with these materials read rather odd to me. We generally have to assume that authors are confident in the work they submit for publication.", "reviewer_2": "The paper investigates the stiffness and electronic properties of a family of high mobility organic semiconductors based on dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) derivatives. The authors focus on understanding the interrelationship between molecular structure, nanoscale mechanical stiffness, and charge carrier mobility in organic semiconductor films.\n\nOne major concern I have with the paper is the relevance of the work – the authors measure the mechanical properties in the z-direction, find that it doesn’t correlate well with the reported charge mobilities because charge mobility is measured in the xy-direction, thus calculate the expected mechanical property in the xy-direction and correlate that to the charge mobility. While they find that there is some correlation, they also ultimately conclude that the charge mobility isn’t as simple as that and is also related to the electronic coupling between molecules. In some ways, these are all expected findings and it is unclear what new knowledge it provides to the field. To make the paper suitable for Nature Communications, I’m wondering if the paper should be more focused on the unique aspects of the work, not on the failed hypothesis/narrative.\n\nIn particular, what must have been an immense amount of work are the AFM measurements. The authors are to be applauded to the attention in detail for their measurements using AFM – it’s a shame that much of the effort and detail has been sidelined to the SI because one can only imagine that it must have been a huge amount of effort to elucidate the details of this measurement. Similarly, it’s a bit of a shame that the simulation is tucked away in the SI.\n\n1. Figure 4b – the y-axis refers to the ab-plane but it might be helpful to refer to it in terms of xyz to be consistent with the discussion. I recognize that they are not quite the same thing but given that the authors themselves state that “unit cells are monoclinic with vertical c-axes being nearly perpendicular to the substrate’s xy-plane, and so very close to the substrate’s z-axis.” I think it may be ok. How were the data for Figure 4b obtained?\n2. Page 14: I found the following statement a bit confusing “This is an indication of the crystallinity of the films”. How does the shift in peak position refer to the crystallinity of the film?\n3. Page 16: I found the first paragraph to be very repetitive compared to what was stated on the previous page.\n4. Can the authors comment on the chiral derivatives more? In line with the theme of the paper, how does the mechanical property reflect in the charge mobility in the two examples investigated?", "reviewer_3": "This work offers an insightful investigation into the origins of stiffness in organic semiconductor (OSC) layers by revealing lattice-dependent mechanical properties through atomic force microscopy (AFM). To achieve this, the authors developed an out-of-plane AFM characterization method tailored for probing mechanical properties. Notably, the results successfully disentangle the effects of crystalline structure on stiffness while elucidating the relationship between surface mechanical properties and carrier transport. This study provides a valuable strategy to guide researchers in exploring surface physical properties and informs molecular design for optimized elastic and electrical performance. There are additional comments which must be addressed for clearer explanation of this manuscript.\n\n(1) As demonstrated in this manuscript, the molecular lattice plays a crucial role in determining the stiffness of OSC thin films. The author analyzed the crystalline unit cell and molecular stacking at the nanoscale (as shown in SI Section 4). However, how can the geometry of an individual molecule on the substrate be confirmed? Specifically, how can the molecular orientation at the AFM tip contact point be determined? Furthermore, does the molecular orientation influence the stiffness, and if so, how?\n(2) Could the domain size, surface roughness, and film thickness influence this analysis, given that the shear stress in AFM measurements can be significantly impacted by the contact edge of the thin film?\n(3) The author demonstrated how the intrinsic factor--molecular structure--influence the stiffness of semiconductor thin films. DNTT and its derivatives exhibit excellent crystalline film quality. Could this approach serve as a universal strategy for other small molecules, such as Naphthalene diimide (NDI) molecules with larger π-conjugated cores?\n(4) It is intriguing that the author links the carrier transport properties with the mechanical properties, showing that stiffer lattices along the charge transport direction contribute to higher charge carrier mobilities. However, it is challenging to directly correlate intermolecular hopping with lattice stiffness. Does the stiffer lattice enhance pi-pi overlap and facilitate more efficient carrier transport pathways?"}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_2": "In this revised version, the authors have carefully addressed the concerns raised by the previous reviewers. The authors have demonstrated careful control of AFM parameters to extract accurate measurements for the mechanical properties and they have validated their results through support from DFT and MD. The technique, in general, would be applicable other organic semiconductors. I believe that the paper is now appropriate for publication.\n\nThings for the authors to consider:\n1. Given that the mechanical property measurement is in the z-direction, have the authors considered correlating that to the charge mobility in the z-direction by, for example, performing SCLC measurements? Alternatively, perhaps use the AFM to obtain conductivity measurements (c-AFM)?\n2. I’m also wondering if there is anyway to extract two different values for the stiffness. One for the aromatic core and one for the alkyl region. I’m naively wondering if as the tip approaches the surface, one could first get the alkyl region value and then the stiff of the aromatic core as the tip approaches the surface more.", "reviewer_3": "The current manuscript, revised by Hwang, deepens the understanding of the nanomechanical properties influenced by the hierarchical structure, through artifact-free AFM technique. The comprehensive characterization and analysis strengthen the significance and reliability of the research, with the systematic analysis contributing to a better understanding of the relationship between stiffness and charge carrier mobility. The systematic analysis contributes to the understand the relationship between the stiffness and charge carrier mobility. There are still several issues that need to be addressed.\n\n1. The authors performed GIXRD measurements on DNTT and its derivatives. Do they have any corresponding 2D diffraction patterns or additional data? In fact, the diffraction results provide a statistical analysis of crystalline stacking in regions exposed to X-ray irradiation. While MD and DFT simulations are included for comparison, the relationship between molecular stacking orientation and mechanical properties needs clearer explanation and further clarification.\n2. The carrier transport properties in this study were assessed through FET devices. Have the authors considered analyzing the electrical performance/carrier transport via conductive-AFM in conjunction with the nanomechanical measurements? This could provide additional insight into the electrical behavior at the molecular level.\n3. In addition, the language expression and structural framework of the article are not very reasonable, which creates difficulties in reading. The author should revise the entire manuscript, optimizing aspects such as formatting and paragraph organization.", "reviewer_4": "The authors present a joint experimental and theoretical investigation of the relationship between molecular structure, packing configuration, and stiffness of molecular crystals. Here, the molecules have a consistent pi-conjugated core structure, and differ by side chains appended symmetrically on the ends of the long axis of the core; the side chains range from hydrogen (unsubstituted), phenyl, and alkyl (with three variations, though the overall lengths are the same). Notably, the side chains do change the tilt of the molecular long axis with respect to the surface and direction of the mechanical force measurements.\n\nThe authors have done a commendable job in responding to the first round of reviewer comments. Indeed, the major focus of the manuscript was changed. However, the authors have not sufficiently addressed what the “noteworthy results” of the work are. That careful measurements have been done, and subtleties that need to be considered when making the measurements are noted, is important work; however, this is not necessarily noteworthy. The outcomes of the physical measurements, and likewise of the DFT and MD simulations that are validated by experiment, are not surprising given the natures of the molecules and substitutions explored. Hence, the overall significance of the work is limited.\n\nThe data analyses are complete, and appear to be interpreted correctly and appropriately. The work does not appear to make spurious claims that go beyond what is measured / calculated. This is a solid paper, and the work is well done."}, "author_response": "Response to Reviewers Comments\nWe begin by thanking the reviewers for taking time to look at the improvements made\nafter the first round of review and for raising further points in need of clarification.\nWe also welcome the suportive comments of the new Reviewer # 4 who has contributed\nto the second round of review.\nWe’ve taken time to carefully address, and incorporate, the suggestions of the reviewers\nduring the second review round. Doing so has improved the quality of our manuscript\nfurther, we believe.\nBelow, the reviewers’ comments are shown in black, and ours in purple.\nAll relevant changes have been highlighted in yellow in the main manuscript.\nReviewer #2 (Remarks to the Author):\nIn this revised version, the authors have carefully addressed the concerns raised by the\nprevious reviewers. The authors have demonstrated careful control of AFM parameters\nto extract accurate measurements for the mechanical properties and they have\nvalidated their results through support from DFT and MD. The technique, in general,\nwould be applicable other organic semiconductors. I believe that the paper is now\nappropriate for publication.\nThings for the authors to consider:\n1. Given that the mechanical property measurement is in the z-direction, have the\nauthors considered correlating that to the charge mobility in the z-direction by, for\nexample, performing SCLC measurements? Alternatively, perhaps use the AFM to obtain\nconductivity measurements (c-AFM)?\nConductive-AFM (C-AFM) is indeed a relevant technique using which both mechanical\nproperties and electronic properties can be consonantly measured along the z-direction.\nIn C-AFM, as the reviewer suggested, SCLC measurements can be performed over the\nscanned area by measuring IV characteristics at every pixel. From these IV\ncharacteristics, the SCLC carrier mobility can be extracted at each pixel in the same way\nwe’ve extracted the stiffness.\nThe suggested combination of SCLC/C-AFM measurements and nanomechanical\nmeasurements is in general a fantastic one but may not be appropriate to the DNTT\nderivatives studied in the current work because of their molecular orientation in the\nlattice combined with the theory’s assumptions. Reasons for this are enumerated below.\nThe connection between electronic properties and mechanical properties in molecular\nsemiconductors that Northrup suggested in Appl. Phys. Lett. 99, 062111 (2011) looks at\ncorrelations in 2D, in the direction of charge transport and perpendicular to the faces of\nmolecules. This is the direction of pi-stacking in the xy-plane in our current case. The out-\nof-plane z-direction sees the DNTT molecular derivatives end-on. In Northrup’s acoustic\ndeformation potential model, two quantities namely the elastic modulus E, and the\ncharge carrier confinement length L , are both directly proportional to the mobility 𝜇. The\neff\ntransport regime considered in the deformation potential theory is the band regime\nwhich needs the charge carriers to be able to delocalize over multiple molecular length\nscales (in molecular crystals, it could be as large as a thousand molecules). While a\ndeformation potential picture could make sense in the xy-transport plane, in the z-\ndirection perpendicular to the main plane of transport, charge carrier transport is not\nexpected to take place within a band regime. This is because the conjugated cores of the\nmolecules along the z-direction are weakly interacting, (i) due to insulating alkyl side\nchains for C8-DNTT and, (ii) because of the peculiar crystal cell which hinder any overlap\nof the conjugated cores. In such a case, the preferential transport regime along the z-\ndirection is likely to take place within the hopping regime. In summary, L cannot be\neff\ndefined for transport along the z-direction because the transport along that direction\ntakes place within the hopping regime. So even though the mobility and modulus may be\nmeasured along that direction using the AFM, the guideline model that relates the two\nquantities will unfortunately not be applicable.\nThe above said, the suggestion to incorporate SCLC/C-AFM measurements in tandem\nwith nanomechanical measurements within molecular systems, appropriately oriented\nto expose the pi-staking direction along the z-axis, is one we’ve taken seriously. Our\nlaboratory is currently in the process of purchasing a new C-AFM module. Using this kit,\nwe will explore nanoelectrical-nanomechanical correlations in select molecular\nmaterials in forthcoming work.\nIndeed, as part of the outlook in our work, we showed that the calculated in-plane\nmodulus from MD + DFT is proportional to the in-plane carrier mobility, suggesting\nconsonance with the predications of Northrup’s deformation potential theory. In future,\none will have to identify and study molecular systems where the theory’s stipulations on\nthe fast transport direction are aligned with AFM’s measurement direction, i.e., along the\nz-axis and normal to the thin film in the current measurement configuration.\n2. I’m also wondering if there is anyway to extract two different values for the stiffness.\nOne for the aromatic core and one for the alkyl region. I’m naively wondering if as the tip\napproaches the surface, one could first get the alkyl region value and then the stiff of the\naromatic core as the tip approaches the surface more.\nWhat the reviewer points out should in principle be possible, but it would require some\nmodelling of how the ensemble of the molecules respond to the presence of the tip, and\nfrom this extract information on how compression of one part of the molecule results in\ncompression of the second part. This is a formidable task that by itself would justify\nanother comprehensive research paper. By just looking at the stiffness as a function of\napplied load using the measured data is in our view not sufficient to allow us to conclude\nhow different parts of the molecule respond to the applied load.\nIn closing, the authors are very thankful for the supportive and thought-provoking\ncomments of Reviewer #2. It has been a fantastic opportunity to be able to evolve our\nresearch article through pleasant and constructive dialogue during this review process.\nReviewer #3 (Remarks to the Author):\nThe current manuscript, revised by Hwang, deepens the understanding of the\nnanomechanical properties influenced by the hierarchical structure, through artifact-\nfree AFM technique. The comprehensive characterization and analysis strengthen the\nsignificance and reliability of the research, with the systematic analysis contributing to a\nbetter understanding of the relationship between stiffness and charge carrier mobility.\nThe systematic analysis contributes to the understand the relationship between the\nstiffness and charge carrier mobility. There are still several issues that need to be\naddressed.\n1. The authors performed GIXRD measurements on DNTT and its derivatives. Do they\nhave any corresponding 2D diffraction patterns or additional data? In fact, the diffraction\nresults provide a statistical analysis of crystalline stacking in regions exposed to X-ray\nirradiation. While MD and DFT simulations are included for comparison, the relationship\nbetween molecular stacking orientation and mechanical properties needs clearer\nexplanation and further clarification.\nIn addition to the in-plane and out-of-plane GIXRD measurements that we showed in the\nreply to the reviewers after the first review round and in the main manuscript, 2D\ndiffraction patterns and grazing incidence diffraction measurements on DNTT, C8-DNTT,\nand diPh-DNTT thin films were published in the supporting information section of a few\nrecent papers of ours. See A. Wittmann et al., Phys. Rev. Lett. 124, 027204 (2020) where\nwe clearly showed both in-plane and out-of-plane ordering within the crystallites of these\nmaterials, in evaporated thin films up to 50 nm, like the films studied in our work.\nAdditional GIWAXS data were shown in the supporting information of E. Selezneva et al.,\nAdv. Mater. 33, 2008708 (2021), where we demonstrated that for films under 100 nm\nthickness, well ordered crystallites with the expected crystallography are guaranteed.\nThe supporting crystallographic information files of C8-DNTT were published in our\npaper, G. Schweicher et al., Adv. Mater. 31, 1902407 (2019) where we looked at charge\ntransport in these evaporated polycrystalline thin films. The crystallographic structures\nof the chiral molecular systems, S- and R- DNTT, were solved in a recent paper, M. Volpi\net al., Adv. Sci. 10, 2301914 (2023). Our current work on the nanomechanical properties\nof these organic molecular systems builds upon the foundational knowledge we’ve\npresented in such articles that detailed their molecular orientation and crystallography\nwithin thin films.\nIn addition to our work, the early seminal papers of others such as T. Yamamoto et al.,\nJACS 129, 2224 (2007) and M. J. Kang et al., ACS Appl. Mater. Interfaces 5, 2331 (2013)\nalso clearly showed both in-plane and out-of-plane ordering within evaporated thin films\nof DNTT, diPh-DNTT, and alkylated-DNTT in agreement with what we observe.\nBelow, we share additional data, namely measurements of real time X-ray reflectivity\n(XRR) as the DNTT-based films grow from a thickness of a few nm to around 20 nm.\nFigure 1 Real-time X-ray reflectivity (XRR) scans during growth, offset for clarity. The film thickness d\nincreases from yellow (d = 4 nm) to blue (d ≈ 20 nm). Data for DNTT and two derivatives are shown in (a) –\n(c).\nThese additional data show how Bragg peaks and crystallinity develop within the\npolycrystalline thin films during growth by thermal evaporation. The data in Figure 1,\ntaken together with our earlier articles documenting GIWAXS data referenced above,\nprovide firm ground for the suggested molecular orientation, crystallographic structure,\nand long-range order within the grains of the polycrystalline films.\nLastly, since our AFM measurements are done on the nanoscale and with a resolution\nthat probes intra-grain mechanical properties within the polycrystalline domains, we\nreckon a convincing connection between molecular stacking orientation,\ncrystallography, and mechanical properties can indeed be drawn.\n2. The carrier transport properties in this study were assessed through FET devices. Have\nthe authors considered analyzing the electrical performance/carrier transport via\nconductive-AFM in conjunction with the nanomechanical measurements? This could\nprovide additional insight into the electrical behavior at the molecular level.\nOn the collective suggestion made by the reviewers during the first round of review, our\nrevised manuscript focuses on the measurement of nanomechanical properties of\nmolecular semiconductors. Its correlation with transport properties is now only a soft\ntouch, presented as an outlook to provide context for the importance of nanomechanics\nto organic electronics research. Within the outlook, theory calculations of molecular\nmechanics combined with carrier transport, both along the in-plane direction of fast\ntransport, support the direct proportionality between stiffness and mobility.\nThis said, we have indeed considered performing conductive AFM measurements in\nconjunction with nanomechanical measurements, but as explained in our reply to\nReviewer #2, above, deformation potential theory suggests that one must probe the\nmechanics along the direction of fast carrier transport, i.e., along the pi-stacking\ndirection. Since the DNTT molecular system stacks pillar-like along the z-direction, with\nits pi-stacking direction in-plane (xy-plane), doing conductive-AFM measurements on\nthe current films along the z-direction will not probe transport in the direction of interest\nto the dictates of deformation potential theory’s guideline.\nWe will take the reviewer’s suggestion of undertaking C-AFM on appropriately oriented\nmolecular semiconductors in future (i.e., where the z-direction involves fast transport\nand not just hopping), to probe both nanomechanics and the SCLC-mobility at every\nnanoscale pixel. We will identify appropriately oriented molecular systems where the pi-\nstacking has a significant component along the z-axis when doing tandem\nnanomechanical and nanoelectrical measurements. Such work will constitute a future\nresearch direction from our group, once we establish the C-AFM measurement\ntechnique in our laboratory next year.\n3. In addition, the language expression and structural framework of the article are not\nvery reasonable, which creates difficulties in reading. The author should revise the entire\nmanuscript, optimizing aspects such as formatting and paragraph organization.\nWe have now carefully gone through the entire manuscript again and have carried out\nmajor revisions to the paragraphs and flow of the article. We have enhanced brevity,\nremoved dead words, carried out formatting in areas where necessary, and reorganised\nparagraphs. Figure 3 has been reduced from the original 12 sub-panel figure to a 9 sub-\npanel figure to enhance the cohesiveness of the story overall. The supplementary\ninformation has absorbed the three subpanels that were moved out of Fig 3. The abstract\nand summary have also been revised for clarity. All major changes have been highlighted\nin the resubmitted version of the manuscript. Should the reviewer have any other specific\nsuggestions, we’d be happy to incorporate them.\nWe thank the reviewer for offering us pointers for improvement through the two review\nrounds. It has been immensely gratifying to see the quality and the overall narrative of\nour work improve through their scientific dialogue and their suggestions.\nReviewer #4 (Remarks to the Author):\nThe authors present a joint experimental and theoretical investigation of the relationship\nbetween molecular structure, packing configuration, and stiffness of molecular crystals.\nHere, the molecules have a consistent pi-conjugated core structure, and differ by side\nchains appended symmetrically on the ends of the long axis of the core; the side chains\nrange from hydrogen (unsubstituted), phenyl, and alkyl (with three variations, though the\noverall lengths are the same). Notably, the side chains do change the tilt of the molecular\nlong axis with respect to the surface and direction of the mechanical force\nmeasurements.\nThe authors have done a commendable job in responding to the first round of reviewer\ncomments. Indeed, the major focus of the manuscript was changed. However, the\nauthors have not sufficiently addressed what the “noteworthy results” of the work are.\nThat careful measurements have been done, and subtleties that need to be considered\nwhen making the measurements are noted, is important work; however, this is not\nnecessarily noteworthy. The outcomes of the physical measurements, and likewise of\nthe DFT and MD simulations that are validated by experiment, are not surprising given the\nnatures of the molecules and substitutions explored. Hence, the overall significance of\nthe work is limited.\nThe data analyses are complete, and appear to be interpreted correctly and\nappropriately. The work does not appear to make spurious claims that go beyond what is\nmeasured / calculated. This is a solid paper, and the work is well done.\nWe are thankful to the reviewer for making time to review our work in this round and for\ntheir kind words.\nOur article is an experimental demonstration of the tunability in the nanomechanical\nproperties of molecular semiconductors based on changes in molecular structure via\nsystematic alkyl sidechain substitution. Although such tunability may intuitively be\nexpected, as the reviewer has rightly pointed out, the effect is subtle and extremely\ndifficult to extract from nanoscale measurements due to various spurious force\ncontributions operating on such small length scales. Only after identifying and removing\nthese spurious contributions is the underlying molecular-scale tailoring of mechanics\nobservable. The notable feat is then the first direct measurement of the impact that alkyl\nsidechain substitution has on the mechanical properties. We have heavily edited the\nabstract and summary of the manuscript to make the significance of the work more\nimmediately apparent."}, {"round": 3, "reviewer_comments": {"reviewer_2": "I believe that the authors have addressed the comments as much as they are able to at this time.", "reviewer_3": "The authors have made appropriate revisions. I recommend acceptance of the manuscript."}, "author_response": "Response to Reviewers Comments\nReviewers’ comments are shown in black, and ours are shown in purple.\nReviewer #2 (Remarks to the Author):\nI believe that the authors have addressed the comments as much as they are able to at\nthis time.\nReviewer #3 (Remarks to the Author):\nThe authors have made appropriate revisions. I recommend acceptance of the\nmanuscript.\nWe thank the reviewers for engaging in a constructive dialogue through the review\nprocess and for being supportive of the final version."}]

Decision letter