Generalised optical printing of photocurable metal chalcogenides

Optical three-dimensional (3D) printing techniques have attracted tremendous attention owing to their applicability to mask-less additive manufacturing, which enables the cost-effective and straightforward creation of patterned architectures. However, despite their potential use as alternatives to traditional lithography, the printable materials obtained from these methods are strictly limited to photocurable resins, thereby restricting the functionality of the printed objects and their application areas. Herein, we report a generalised direct optical printing technique to obtain functional metal chalcogenides via digital light processing. We developed universally applicable photocurable chalcogenidometallate inks that could be directly used to create 2D patterns or micrometre-thick 2.5D architectures of various sizes and shapes. Our process is applicable to a diverse range of functional metal chalcogenides for compound semiconductors and 2D transition-metal dichalcogenides. We then demonstrated the feasibility of our technique by fabricating and evaluating a micro-scale thermoelectric generator bearing tens of patterned semiconductors. Our approach shows potential for simple and cost-effective architecturing of functional inorganic materials.

material and are likely due to typical experimental errors when measuring conductivity and Seebeck. The authors should remove this claim or include further studies such as DSC measurements to support it.
Line #260-261: can the author clarify why materials with high carrier concentration show an increase of S with temp? A reference would be useful too.
Line #274: Can the authors elaborate on this statement. According to the Pisarenko relation, S should go up for lower n for highly doped semiconductors, which are usually employed for thermoelectrics (DOI: 10.1039/C1EE02612G). Is this what the authors mean? In the reference provided, the opposite is claimed because the authors of the referred paper compared an intrinsic semiconductor with a doped one.
Line #298: 3D devices are only potentially possible, not demonstrated.
Line #311: I suggest changing for 3-steps (or 2-steps if sintering is considered as a post-processing step) as exposure, and removal of green material are needed.
Figure 2: Some of these panels do not provide any extra relevant info (a,b, and c show similar results, d and e also). Therefore they are redundant and they could be removed or transferred to the SI to make the main text more focused. The manuscript reported a very interesting concept of optical printing of photocurable metal chalcogenides. Optical printing has been mostly used for printing polymers or their composites. The concept reported in this work opens new opportunities of using optical printing to print inorganic semiconductors. I would like to recommend the publication of this article after the authors address the following comments: 1. The reported charge carrier mobility and conductivity seems still considerably lower than the best reported values of the same materials made by other techniques. The authors should provide some explanations why the property is still relatively inferior using the optical printing method. 2. There are many printing methods available to print the inorganic inks. I suggest the authors provide a performance comparison of the optical printing vs. other printing methods in terms of printing speed, spatial resolution, properties of printed materials, etc. 3. The authors demonstrated an in plane TE device. The optical printing method seems a very good method to fabricate cross-plane TE devices. The cross-plane TE devices are much more applicable for energy harvesting devices than in plane TE devices. If possible, the authors may attempt to demonstrate some cross-plane TE devices, which can make this work even more appealing.

EP^[ZY^P _Z _SP ]PaTPbP]^j NZXXPY_T
he followings are the responses to the reviewers' comments for the manuscript J*8?8B4=<C87 @AD<64= AB<?D<?: @9 A;@D@6EB45=8 >8D4= 6;4=6@:8?<78C$K 9ZXXPY_ -8 Line #26: Although the cross-section of the printed shapes can be altered throughout the thickness direction (Fig. 2f), the authors should be more conservative when claiming 3D printing. I think the process is potentially compatible with 3D printing but the thickness demonstrated corresponds still to the thin-film domain (0.5 um). Also, the TEG demo is in-plane. Therefore, no real 3D architecture has been shown. The claim of 3D printing should be replaced with "compatibility with 3D printing" or "potential for 3D printing", etc.
Response: T^Ziik^\bZm^ma^k^ob^p^kzl valuable comment. T^Z`k^^pbma ma^k^ob^p^kzl \hff^gm that the claim of x1A ikbgmbg`y lhf^paZm exaggerated our printing process in this study. To overcome 9ZXXPY_ .8 Line #43: Since the authors have similar papers based on DIW, it would be relevant to introduce here a brief comparison between these two 3D printing methods.
Response: T^Ziik^\bZm^ma^k^ob^p^kzl _knbm_ne \hff^gm, As the reviewer commented, our group have studied the DIW 3D printing process for fabricating various types of inorganic electronic materials. The DIW is an extrusion-based 3D printing method to create meso-and micro-scales architectures. In the DIW, the liquid-iaZl^xbgdy bl ]bli^gl^] hnm h_ lfZee ghsse^l ng]er controlled flow rates and deposited along digitally defined paths to fabricate 3D structures layer-by-layer A1 . Recently, by developing the viscoelastic inks containing functional inorganics, this method has been extensively utilized for fabricating various types of functional inorganic materials. This is the stark contrast to the optical 3D printing techniques such as digital light processing (DLP) and stereolithography, which suffer from the critical issue of limited printable materials, thus their applications in the electronic and energy fields are restricted.
Accordingly, we included the following sentences on the revised manuscript (page 3). Response: T^Ziik^\bZm^ma^k^ob^p^kzl mahn`am_ne \hff^gm, Our group reported soluble telluridebased molecular Sb2Te3 precursors. which can be used as the inks for the current DLP printing A2 . The Sb2Te3 precursor was synthesized by the multi-step processes of the synthesis of polymeric Sb2Te3 precursor and the subsequent superhydride and tri-n-octylphosphine (TOP) treatments (Fig. A2).
Although this final precursor could generate the high-quality thin films by the spin coating process, the complicated synthetic route for the Sb2Te3 precursor requires tight engineering in every step for ensuring the composition and purity of precursors. We have tried to use this precursor for the optical printing process but we concluded that this precursor was not suitable for the current optical printing process because the mixing step with the photoacid generator (PAG) to provide the photoreactivity was found to easily degrade the composition of the Sb2Te3 precursor.
Moreover, the limited solubility of the Sb2Te3 precursor in the DMSO and acetonitrile made it difficult to use this precursor in the current optical printing process that requires the relatively high concentration of precursor. Accordingly, we chose the Cu2S and SnSe2 as thermoelectric materials for fabricating power generating devices.   Response: We apologize for the confusing sentence in the previous manuscript. The printed 2D Cu2S samples was annealed at 723 K and 2D SnSe2 samples was annealed at 573 K. To make it clear, we corrected the previous sentence on the revised manuscript, as follows (page 11). xE^k^* p^_Z[kb\Zm^] 0A @n0P Zg] PgP^0 lZfie^l nlbg`ma^AIM f^mah] Zg] ma^g Zgg^Ze^] @n0P Zm 501 _hk 3* /.* Zg] /3 fbg* Zg] PgP^0 Zm 351 H _hk 5* /.* Zg] /3 fbg* k^li^\mbo^er,y 9ZXXPY_ --8 Line #216: Can you add evidence of this sentence?: "The annealing temperatures were selected after considering the thermal stability of the materials to conserve their stoichiometric composition and microstructural integrity".
Response: T^Ziik^\bZm^ma^k^ob^p^kzl mahn`am_ne \hff^gm. To address this issue, we systematically annealed the SnSe2 and Cu2S samples at different temperatures and characterised them by the XRD and SEM analysis. The SEM images of SnSe2 samples (Fig. A5) shows that the microstructures were not changed significantly. However, the SnSe phase (p-type) was started to detected in the XRD patterns at 623 K and the SnSe2 phase was fully transformed to the SnSe phase at 673 K. This composition transition was well-known to be attributable to the evaporation of Se A3 . Since SnSe crystal  Fig. 14) shows that the microstructures were not changed significantly. However, the SnSe phase (p-type) was started to detected in the XRD patterns at 623 K and the SnSe2 phase was fully transformed to the SnSe phase at 673 K. This composition transition was well-known to be attributable to the evaporation of Se 20 . Since SnSe crystal generally exhibit the p-type properties, we chose the annealing temperature of 573 K to conserve the n-type character of the SnSe2 sample. On the other hand, regardless of the annealing temperatures, all Cu2S samples shows the smooth film with good coverage, as shown in the SEM images (Supplementary Fig. 15). However, the XRD patterns of the samples annealed at 623 K and 673 K corresponded to the Cu1.8S bulk reference, while the samples annealed at higher temperatures showed the Cu1.96S phase ( Supplementary Fig. 15b). In general, Cu1.96S crystal is known to exhibit significantly higher thermoelectric properties than those of Cu1.8S 21 . Accordingly, we chose the annealing temperature of 723 K to obtain the Cu1.96P iaZl^,y 9ZXXPY_ -.8 Line #222: Could the authors clarify how the evaporation of S leads to fewer Cu vacancies?
Response: We Ziik^\bZm^ma^k^ob^p^kzl \hglmkn\mbo^\hff^gm hg hnk fZgnl\kbim, @hii^k chalcogenides Cu2-xX (X = S, Se or Te) have been studied as promising thermoelectric materials. The electrical and functional properties of copper chalcogenides depend on not only the crystal structures but stoichiometry of composition due to the carrier concentration. Because the Cu vacancy in copper chalcogenides act as the hole donor, the hole carrier concentration is generally increased with increasing the copper deficiency from Cu2X to Cu2-xX. Lin et al. reported the decreased carrier concentrations of Cu2Se thin films with increasing the heat treatment temperatures A5 . The author claimed that the higher annealing temperature leads to the lower Se contents, as a result, which reduces the self-doping effect of the copper deficiency and decrease the hole concentration (Fig. A8). Likewise, in our printed Cu2S samples, the annealing time dependences of carrier concentrations could result from the leaving chalcogen (in our case, the S evaporation) that reduces the self-doping effect caused by the Cu deficiency and decreases carrier concentrations. 9ZXXPY_ -48 Line #311: I suggest changing for 3-steps (or 2-steps if sintering is considered as a postprocessing step) as exposure, and removal of green material are needed.
Response: Thank you for the kind comment. We edited all grammatical errors and typos in the revised manuscript.
9ZXXPY_ .,8 Figure 2: Some of these panels do not provide any extra relevant info (a,b, and c show similar results, d and e also). Therefore they are redundant and they could be removed or transferred to the SI to make the main text more focused.
Response: W^`^g^kZeer Z`k^^pbma ma^k^ob^p^kzl \hff^gm maZm Cb`, \hgmZbgl k^]ng]Zgm iZg^el, We moved the Fig. 2e to the Supplementary Information. However, Fig. 2a, b, and c    9ZXXPY_ .8 There are many printing methods available to print the inorganic inks. I suggest the authors provide a performance comparison of the optical printing vs. other printing methods in terms of printing speed, spatial resolution, properties of printed materials, etc.
Response: T^Ziik^\bZm^ma^k^ob^p^kzl oZenZ[e^\hff^gm, >l the reviewer suggested, we summarized the recently reported printing methods for inorganic materials in terms of printable materials, printing speed, spatial resolution, processing steps, and the properties of materials (Table B2). Our printing process are advantageous for the printing speed and simplicity in the printing process compared with other methods including inkjet printing, transfer printing, extrusion-based 3D printing, and other optical printing processes. In addition, the properties of the printed materials, especially electrical properties, are comparable to or even higher than the films printed by other process except the direct optical lithography, which adapts the typical photolithographic techniques on the thin films for patterning. In addition, our digital light processing (DLP)-based optical printing, which creates solid architectures directly from liquid resin inks using automatically patterned digital masks and light exposure, OZP^Yj_ YPPO XL^V []ZO`N_TZY( XL_P]TLW deposition, and subsequent lift-off process, thus which can additionally be beneficial for reducing the processing cost and difficulty. 9ZXXPY_ /8 The authors demonstrated an in plane TE device. The optical printing method seems a very good method to fabricate cross-plane TE devices. The cross-plane TE devices are much more applicable for energy harvesting devices than in plane TE devices. If possible, the authors may attempt to demonstrate some cross-plane TE devices, which can make this work even more appealing.
Response: T^Ziik^\bZm^ma^k^ob^p^kzl oZenZ[e^ln``^lmbhg, We have tried our best to fabricate the cross-plane thermoelectric device by the current DLP printing process. Fig. B1a shows the schematic illustration of the entire fabrication process, in which we used one pair of the DLP-printed Cu2S and SnSe2 films as p-type and n-type thermoelectric semiconductors. Since the current printing process is not applicable to electrode materials, we used the deposited Au and Ag paste layers as the bottom and top electrodes, respectively. Qa^]^ob\^k^lblmZg\^h_ t/3. pZl h[l^ko^] Zm khhf m^fi^kZmnk^, We measured the power-generating performance of the fabricated generator by heating the bottom with can appreciate that a great effort has been done to improve the quality of the manuscript. There are only a few small details that still need clarification, after this the paper is, in my opinion, ready for publication.

Response:
We appreciate the reviewer's valuable time and effort in evaluating our manuscript. We truly agree with the reviewer's comments about the need for the additional characterisation and clarifications of the process and materials. We sincerely addressed all the comments and believe that these revisions made a significant improvement in the quality of the manuscript. We thank the reviewer again for the positive comment.
/@>>8?D ': Comment#2: The authors add a good comparison but it fails to show the advantages of DLP vs DIW. If DIW is better in every aspect, why use DLP? Could the authors mention explicitly some advantages of DLP vs DIW? Maybe resolution or throughput? "This is the stark contrast" should be -> "This is in stark contrast" Response: We appreciate the reviewer's valuable comment. Optical printing methods such as DLP and SLA can be advantageous for high-resolution, high-throughput, and large-scale printing, compared with other printing methods, especially the DIW. Accordingly, we included the following sentences in the revised manuscript (page 3). Also, we have tried to edit all grammatical errors in the entire manuscript.
"Nevertheless, the optical printing methods of DLP and SLA can be advantageous for high-resolution, highthroughput, and large-scale printing, which can offer great potential for patterning high-performance inorganic materials." /@>>8?D (: Comment #12: In their response, the authors claimed that "the hole carrier concentration is generally increased with increasing the copper deficiency from Cu2_X to Cu_{2-x}X." I agree with this statement but this contradicts what is written in the text: "decreasing the number of Cu vacancy defects because of increasing the copper deficiency from Cu_2X to Cu_{2-x}X". More Cu deficiency should lead to more vacancies, right? In this work, if S is evaporated, then the Cu content should be actually increasing (compared to S, i.e. Cu_2X becomes Cu_{2+x}X) and this will lead to reducing the vacancies and the carrier concentration. This will agree with the results but the explanation provided by the authors seems wrong then... Did not I understand it well?

Response:
We apologize for the confusing description in the previous manuscript. As the reviewer commented, the S evaporation lead to the increase of the Cu content, reducing the Cu vacancies and hole concentrations.
Accordingly, we replaced the previous sentences with the following sentences in the revised manuscript (page 11).
"Because the intrinsic defects of Cu vacancies in Cu2S act as hole donors, longer annealing times could promote the evaporation of S, eventually decreasing the hole concentrations by increasing the relative Cu contents and reducing the Cu vacancy defects 53 ." /@>>8?D ): Comment #15: The explanation is convincing, but the sentence "which shows the shift of the peak temperatures" is not clear. Please rephrase. Do you mean the shift of the peak value of the Seebeck coefficient to a different temperature?

Response:
We apologize for the unclear sentences in the revised manuscript. As the reviewer noted, we intended to describe that the peak value of the Seebeck coefficient could shift to higher temperature ranges with increasing the carrier concentration.
Accordingly, we replaced the previous sentences with the following sentences in the revised manuscript (page 14).
"Accordingly, these materials usually show the positive temperature dependences in a wide temperature range. This behaviour is generally reflected in the temperature-dependent Seebeck coefficients of materials, which shows