# Diamonds for Life: Developments in Sensors for Biomolecules
## 2 Chemical Vapour Deposited Diamond Thin Films  
Diamond thin films can be deposited by CVD on a variety of non-diamond substrates in a carbon-containing atmosphere saturated with hydrogen. The latter plays a pivotal role: on the one hand, hydrogen atoms generate radicals on both the hydrocarbon source and the diamond substrate; on the other, they etch \(sp^{2}\)-hybridised carbon (graphite) away at a much higher rate than \(sp^{3}\)-hybridised carbon (diamond), which results in the growth of high-quality diamond films. Hydrogen atoms further stabilise the dangling bonds at the diamond surface [9], being at the genesis of some of the most unique properties of diamond, such as high chemical inertness, large potential window, hydrophobicity, and negative electron affinity (NEA).  
Natural diamond is an insulator. When films are deposited by CVD in a hydrogen-rich environment, they exhibit a property deemed \"surface conductivity\" [10]. This happens because carbon is slightly more electronegative than hydrogen, resulting in the formation of a dense surface dipole layer with slightly negative charged carbon atoms (C\({}^{-}\)) and slightly positive charged hydrogen atoms (H\({}^{+}\)), which causes an electrostatic potential step \(\Delta\)V perpendicular to the surface over a distance of the order of the C-H bond length, i.e., 1.1 A. Energy variation over this dipole is in the range of 1.6 eV [1, 11].  
The conductivity of diamond can be further tailored by including charge-carrier elements, such as boron, nitrogen, and phosphorous, into its lattice. Doping diamond with boron makes it become a \(p\)-type semiconductor, with an activation energy (\(E_{\text{a}}\)) of 0.37 eV, while \(n\)-type behaviour can be obtained by doping diamond with nitrogen, \(E_{\text{a}}=1.7\) eV, or phosphorous, \(E_{\text{a}}=0.56\) eV [12].  
The surface of a diamond thin film is typically polycrystalline, with grain sizes within the micrometric range. The versatility of the CVD technique allows for the preparation of multi-layered materials, where doped and undoped layers can be deposited one on top of each other simply by changing the composition of the gas environment, allowing the fabrication of films with tailored properties. An as example, the electrical properties of the material stack can be improved by growing a thin layer of non-doped nanocrystalline diamond on top of a boron-doped polycrystalline film (Figure 2).  
When the undoped nanolayer is deposited over a base layer of conducting polycrystalline diamond, the combined material can be employed as an electrode with improved selectivity for small biomolecules. A literature example reports the deposition of nanocrystalline diamond over boron-doped diamond using two hours of deposition at 20 Torr, 50 sccm CH\({}_{4}\), 200 sccm H\({}_{2}\), 2 sccm C\({}_{2}\)H\({}_{2}\), a substrate bias of \(-\)200 V, and a substrate temperature of 800 \({}^{\circ}\)C) [13].  
More recently, [PERSON] et al. showed that it is possible to produce boron-doped diamond (BDD) starting from non-doped diamond and using a secondary deposition to perform the boron doping. As expected, BDD samples presented electrochemical properties that the diamond film did not have. As the thickness of the doped layer increased (by increasing the time of doping), there was an increase in the potential window, but there was no change in electrochemical activation, because this is a surface phenomenon [14].