\documentclass{standalone}
\usepackage{tikz}
\usepackage{tikz-qtree}
\usepackage{xcolor}
\usetikzlibrary{arrows}
\usepackage{fontspec}
\setmainfont{Helvetica}
\begin{document}
\begin{tikzpicture}
\tikzset{level distance=30pt}
\tikzset{every  leaf node/.style={text=blue,font=\bf}}
\tikzset{every internal node/.style={text=darkgray,font=\bf}}
\tikzset{edge from parent/.append style={thick}}

\Large

\Tree[.DET [.NP \_\_\_ ] [.POSS 's ] ]


\end{tikzpicture}
\end{document}
