## 4 Conclusions  
A one-step procedure based on the flame synthesis of carbon nanoparticles and their direct deposition by thermophoresis on a substrate with interdigitated gold electrodes has been adopted in this study. Both the morphological and structural characteristics of so prepared carbonaceous films have been characterized by AFM and Raman spectroscopy. The electrical resistance as a function of the temperature has been investigated from ambient temperature up to nearly 400 K.  
A monotonic decrease of the film resistance with temperature increase is observed, which is typical of a nonmetallic behavior. The change of resistivity with respect to temperature is described by a negative temperature coefficient of resistance (TCR), with absolute value higher than 100 \(\times\) 10\({}^{-4}\) K\({}^{-1}\). A functional relationship between resistance and temperature, which is necessary for practical applications, has been finally derived. A good fit is obtained with a fifth order equation.  
The sensitivity of the CNP film's electrical response to temperature changes, together with the ease of material microfabrication processing and biocompatibility of the carbon material, make these films ideal for temperature sensing in many environments.  
In a future work, the study will be extended to higher temperatures to investigate the maximum working temperature before structural damage occurs. Furthermore, different films will be investigated as function of the main synthesis parameters (such as fuel kind and equivalence ratios, flame temperatures, particle residence time in flame, etc.) to optimize the sensor response to the temperature and its stability.  
The technique to produce carbon nanostructured thin films presented in this work is a reliable, low-cost procedure that does not make use of solvent nor toxic substances. Furthermore, the films can be deposited on a variety of substrates and with arbitrary shapes.  
The flame synthesized CNP films can be easily applied to developing temperature sensors embedded in different materials with different geometries. This is a promising technology open to novel applications such as, for instance, smart packaging and wearable biosensors.  
Figure 8: (a) Functional relationship between resistance and temperature, 1/T vs. ln(R/R\({}_{0}\)) with the resistance evaluated R\({}_{0}\) at ambient temperature. (b) Residuals versus T.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12157714/s1](https://www.mdpi.com/article/10.3390/app12157714/s1)]([https://www.mdpi.com/article/10.3390/app12157714/s1](https://www.mdpi.com/article/10.3390/app12157714/s1)), Figure S1: particle diameter; Figure S2: XPS C1s spectrum; Figure S3 energy loss spectrum; Figure S4: UPS valence band spectrum; Figure S5: Tauc plot; Figure S6: I-V curves. References [29, 33, 40] are cited in the supplementary materials.
* [2]**Author Contributions:** Conceptualization, P.M., M.C., A.A. and A.D.; methodology, P.M. and M.C.; investigation, P.M., G.D.F. and M.C.; data curation, P.M., G.D.F., A.A. and M.C.; writing--review and editing, P.M., G.D.F., M.C., A.A. and A.D.; funding acquisition, P.M. and A.D. All authors have read and agreed to the published version of the manuscript.
* [3]**Funding:** This research was conducted under the PRIN project 2017 PJ5 XXX: \"MAGIC DUST\" of the Italian Ministero dell'Universita e della Ricerca, MUR.
* [4]**Institutional Review Board Statement:** Not applicable.
* [5]**Informed Consent Statement:** Not applicable.
* [6]**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.
* [7]**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.