Our initial approach for depositing organic inks was to attempt DPN with octadecanethiol (ODT), a classic ink for DPN that reproducibly transfers to the substrate. However, it was found that the ink on the DPN tip would invariably evaporate in the load lock chamber (~10−7 Torr) leaving insufficient coverage for observable deposition. Evaporation is readily observed visually since the ink leaves a haze on the tip that is absent after placing in a load lock chamber. This anecdotal observation was more rigorously examined by creating a sample that mimicked the DPN tip surface chemistry: A silicon oxide on a silicon chip that was coated by holding it over ODT in a scintillation vial heated to 65 °C, for 30 min. This procedure produced an ODT film that was 20 nm thick (measured by ellipsometry). After placing the chip briefly under vacuum in a load lock chamber (~10−7 Torr), no ODT film was detectable. Additional attempts with less volatile inks – such as eicosanethiol – yielded similar results, leading us to conclude that typical inks used in conventional DPN cannot be used for DPN under vacuum.
While alkanethiols could not be deposited, we found that heated probes would retain and deposit polymer in UHV. For this work, we chose the polymer to be poly(3-dodecylthiophene) (PDDT), a conducting polymer that has found widespread usage in organic electronics () [13
]. PDDT is also interesting because it becomes highly ordered, forming self-assembled layers on a silicon surface [14
], when it is properly annealed. This ordering increases its ability to conduct current after electron beam exposure [15
The probe temperature was controlled by applying current through the probe heater [16
]. One of the advantages of UHV tDPN is the lower melting point of inks under UHV. Because the molar volume of PDDT is lower in solid form than in liquid form, thermodynamics indicate that its melting point should drop as the surrounding pressure is lowered. Thus, while PDDT routinely deposits at its melting point of 120 °C in air, we observed that the writing temperature of PDDT could be decreased down to ~100 °C in UHV. As a result, the temperature window between melting and thermal decomposition of PDDT (175 °C in air) widens, thereby enabling greater control of line widths and thicknesses deposited in UHV. The lower deposition temperature also reduces the risk of thermal damage when applied to pre-fabricated devices.
While heating the probe to the vacuum melting temperature of the PDDT, the tip was rasterized across the “as is” native oxide Si substrate at different speeds. We found that monolayer-by-monolayer control of the film thickness, as previously established under nitrogen, is also possible under UHV. shows two polymer nanowire lines written at different speeds. Assuming a thickness of 2.6 nm for each PDDT monolayer as previously determined by XRD [14
], the polymer deposited by the probe moving at 20 µm/s was only a single monolayer thick, with the structure written at 8 µm/s being four monolayers thick. The widths of the deposited polymer structures were 280 nm at 20 µm/s and 303 nm at 8 µm/s, with the width principally determined by the relatively blunt silicon tip. Note that recent advances – where the tips remain sharp due to a coating of wear-resistant diamond – readily show line thicknesses of 40 nm [17
]. The line width and heights were measured as a function of the probe speed (). The heights of the deposited polymer structures roughly decrease as the inverse square root of the scan speed. The widths of the deposited structures decrease monotonically with the scan speed but do not show a clear power law relationship. When patterning under ambient conditions, dimensional control may be achieved by varying the tip temperature; however, the tip temperature was fixed in UHV to limit the number of experimental parameters.
Figure 2 Orientations of UHV deposited polymer. (a) PDDT typically organizes in such way that the polymer is oriented normal to the surface with a monolayer height of 2.6 nm. (b) Deposition of polymer at different speeds on a non-UHV prepared substrate showing (more ...)
The polymer deposit heights and widths of PDDT deposited onto Si substrate (non-UHV prepared) as a function of scanning speed. Both the height and width decrease monotonically with tip speed.
Polymer nanostructures were also written on atomically clean and flat Si(001)-2×1 () where monoatomic steps are clearly visible. Interestingly, we found that surface chemistry of the silicon substrate had a major effect on the apparent structure of the deposited polymer as determined by the monolayer film thickness. On the native oxide surface, PDDT self-assembles in such way that the side chains are perpendicular to the surface (), as typically observed for PDDT deposited on non-UHV prepared surfaces under ambient conditions [18
]. The upright orientation is due to the hydrophobic alkyl side chains minimizing their exposure to the hydrophilic oxide substrate. In contrast, PDDT written on Si(001)-2×1 has a film thickness of ~0.4 nm, corresponding to polymer side-chains oriented parallel to the surface, as illustrated in . Note that the thickness of our films lies intermediate to values reported previously for PDDT on other substrates. Scifo et al. used STM to measure the thickness in UHV of a PDDT film drop cast on highly oriented pyrolytic graphite (HOPG) and reported a film thickness of 0.24 ± 0.04 nm [19
]. In contrast, Terada et al. [20
] reported poly(3-hexylthiophene) (P3HT) on H-terminated Si(100) in UHV to be 0.5 nm thick. Our measured value is closer to the 0.4 nm intermolecular spacing measured for thick films of PDDT [14
]. In the prior STM measurements, the measured thickness is a convolution of the topographic height and electronic properties of the polymer film, complicating the comparison. However, the polymer’s lying flat strongly suggests that alkyl side chains must interact more favorably with the silicon surface than with the oxide surface and so has a significant impact on the observed molecular film thickness.
Figure 4 (a) Deposition onto the UHV prepared Si substrate in UHV shows the polymer lying on its side. (b) Polymer deposited across a Si step edge an atom thick. (c) The cross section [pale blue line in (b)] shows that the polymer thickness is 0.4 nm, indicating (more ...)